<?xml version="1.0" encoding="us-ascii"?><InstanceReport xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"><Version>2.4.0.8</Version><ReportLongName>111 - Disclosure - Future Interest Agreement</ReportLongName><DisplayLabelColumn>true</DisplayLabelColumn><ShowElementNames>false</ShowElementNames><RoundingOption /><HasEmbeddedReports>false</HasEmbeddedReports><Columns><Column FlagID="0"><Id>1</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><CurrencyCode /><FootnoteIndexer /><hasSegments>false</hasSegments><hasScenarios>false</hasScenarios><MCU><KeyName /><CurrencySymbol /><contextRef><ContextID>P01_01_2013To06_30_2013</ContextID><EntitySchema>http://www.sec.gov/CIK</EntitySchema><EntityValue>0000926843</EntityValue><PeriodDisplayName /><PeriodType>duration</PeriodType><PeriodStartDate>2013-01-01T00:00:00</PeriodStartDate><PeriodEndDate>2013-06-30T00:00:00</PeriodEndDate><Segments /><Scenarios /></contextRef><UPS /><CurrencyCode /><OriginalCurrencyCode /></MCU><CurrencySymbol /><Labels><Label Key="CalendarSupplement" Id="0" Label="6 Months Ended" /><Label Key="Calendar" Id="1" Label="Jun. 30, 2013" /></Labels></Column></Columns><Rows><Row FlagID="0"><Id>1</Id><IsAbstractGroupTitle>true</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><Level>1</Level><ElementName>ck0000926843_FutureInterestAgreementAbstract</ElementName><ElementPrefix>ck0000926843_</ElementPrefix><IsBaseElement>false</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsCalendarTitle>false</IsCalendarTitle><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><FootnoteIndexer /><Cells><Cell FlagID="0" ContextID="" UnitID=""><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText /><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell></Cells><ElementDataType>xbrli:stringItemType</ElementDataType><SimpleDataType>string</SimpleDataType><IsTotalLabel>false</IsTotalLabel><UnitID>0</UnitID><Label>Future Interest Agreement [Abstract]</Label></Row><Row FlagID="0"><Id>2</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><Level>2</Level><ElementName>ck0000926843_ManagementAgreementTextBlock</ElementName><ElementPrefix>ck0000926843_</ElementPrefix><IsBaseElement>false</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsCalendarTitle>false</IsCalendarTitle><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><PreferredLabelRole>terseLabel</PreferredLabelRole><FootnoteIndexer /><Cells><Cell FlagID="0" ContextID="P01_01_2013To06_30_2013" UnitID=""><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText>              &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt;MARGIN: 0pt 0px; FONT: 10pt Times New Roman, Times, Serif "&gt;  &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt;MARGIN: 0pt 0px; FONT: 10pt Times New Roman, Times, Serif"&gt;  &lt;u&gt;Note 5:&lt;/u&gt;&lt;u&gt;Future Interest Agreement&lt;/u&gt;&lt;/div&gt;    &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt;MARGIN: 0pt 0px; FONT: 10pt Times New Roman, Times, Serif"&gt;  &amp;#160;&lt;/div&gt;    &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt;TEXT-ALIGN: justify; MARGIN: 0pt 0px; FONT: 10pt Times New Roman, Times, Serif"&gt;  &lt;font style="FONT-SIZE: 10pt"&gt;In March 2003, NPAMLP, NPAEP and  PVPG, entered into an Agreement, effective as of &lt;font style=" FONT-SIZE: 10pt"&gt;January 1, 2003&lt;/font&gt; (the &amp;#8220;2003  Agreement&amp;#8221;), in which NPAEP and PVPG agreed with NPAMLP to  modify the terms of Wrap Mortgages held by NPAEP and PVPG. &lt;font  style=" "&gt;The terms of the 2003 Agreement provided that NPAEP and  PVPG: (a) reduce to &lt;font style=" FONT-SIZE: 10pt"&gt;4.1&lt;/font&gt;% per  year the annual interest rate payable on any NPAEP Wrap Note or  PVPG Wrap Note that bears a stated annual interest rate in excess  of that amount (the reduction in the interest rate was evaluated by  NPAMLP in accordance with FASB authoritative guidance, and was  determined not to be a substantial modification of terms as defined  therein); (b) remove certain of the properties secured by the NPAEP  and PVPG Wrap Mortgages from the burden of the cross-default and  cross-collateralization provisions currently contemplated by the  Restructuring Agreement effective as of January 1, 1990 by and  among MLPG, NPAMLP, National Property Analysts, Inc. and others;  and (c) agree to release the lien of the Wrap Mortgages from the  Properties upon a sale of or the agreement of a leasehold estate in  any Property prior to the maturity of the applicable Wrap Note. In  consideration for the above, NPAMLP modified the NPAEP Wrap  Mortgages and the PVPG Wrap Mortgages to provide that (i) there is  an event of default under the applicable NPAEP Wrap Mortgages or  PVPG Wrap Mortgages, as the case may be, if a judgment or other  lien is entered against the title or lease-holding entity thereby  entitling NPAEP or PVPG, as the case may be, to avail itself of the  post-default rights or remedies under the relevant security  document; and (ii) for cross-default and cross-collateralization  among certain partnerships comprising NPAMLP. In addition NPAMLP  shall execute and deliver to NPAEP or PVPG, as the case may be, a  currently recordable deed of future interest (or assignment of  future leasehold interest) sufficient to convey to NPAEP or PVPG,  as the case may be, all of NPAMLP&amp;#8217;s right, title, interest  and estate in and to its fee or leasehold interest in the  encumbered properties effective upon the maturity on December 31,  2013 of the NPAEP Wrap Mortgages and the PVPG Wrap Mortgages unless  the Wrap Mortgages have previously been paid in  full.&lt;/font&gt;&lt;/font&gt;&amp;#160;&lt;/div&gt;    &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt;TEXT-ALIGN: justify; MARGIN: 0pt 0px; FONT: 10pt Times New Roman, Times, Serif"&gt;  &amp;#160;&lt;/div&gt;    &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt;TEXT-ALIGN: justify; MARGIN: 0pt 0px; FONT: 10pt Times New Roman, Times, Serif"&gt;  The Managing General Partner believes that the execution and  delivery of the 2003 Agreement had the following effects for NPAMLP  as a result of the reduction in the annual interest rate on the  NPAEP Wrap Notes and the PVPG Wrap Notes (i) NPAMLP realized  reductions in interest that it otherwise would have been obligated  to pay during the period between January 1, 2003 and December 31,  2013 when these loans mature and (ii) NPAMLP has been able to  allocate a greater portion of its available cash flow to principal  repayments.&lt;/div&gt;    &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt;TEXT-ALIGN: justify; MARGIN: 0pt 0px; FONT: 10pt Times New Roman, Times, Serif"&gt;  &amp;#160;&lt;/div&gt;    &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt;TEXT-ALIGN: justify; MARGIN: 0pt 0px; FONT: 10pt Times New Roman, Times, Serif"&gt;  The Wrap Mortgages owned by NPAEP or PVPG are due and payable in  substantial &amp;#8220;balloon&amp;#8221; amounts on December 31, 2013.  Assuming no sales of Properties by NPAMLP in the interim period  (through 2013) the projected balance due for all of the Wrap  Mortgages at December 31, 2013 is expected to approximate $&lt;font  style=" FONT-SIZE: 10pt"&gt;110,000&lt;/font&gt;. As described above, in  return for the reduction in interest rate and other consideration  set forth above, including the satisfaction of the Wrap Mortgages  due on December 31, 2013, NPAMLP&amp;#8217;s Managing General Partner  has agreed to deliver deeds of future interest and assignments of  leasehold interest, to be recorded currently, effective December  31, 2013, to NPAEP and PVPG. NPAMLP&amp;#8217;s Managing General  Partner has determined that it is in the best interests of NPAMLP  and its partners to do so. The effect of these deeds and  assignments will be to facilitate a transfer of fee and leasehold  ownership to the holders of the Wrap Mortgages at maturity (unless  the Wrap Mortgages have been previously paid in full).  Notwithstanding the foregoing, NPAEP and PVPG have agreed in the  2003 Agreement to (a) release the liens of the Wrap Mortgages and  (b) deliver such deeds of future interest, assignments of leasehold  interests, or other documents or instruments as are necessary to  facilitate or effect such sales of the Properties prior to December  31, 2013 as the Managing General Partner shall otherwise deem  desirable. The costs incurred arising from the recordation of any  of the documents described in the 2003 Agreement will be borne by  NPAEP. The Managing General Partner believes that the result of the  forgoing actions taken pursuant to the 2003 Agreement will preserve  all rights of the Limited Partners under the Restructuring  Agreement, including their right to share in certain sales proceeds  or cash flows prior to maturity of the Wrap Mortgages.&lt;/div&gt;    &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt"&gt;  &amp;#160;&lt;/div&gt;    &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt;TEXT-ALIGN: justify; MARGIN: 0pt 0px; FONT: 10pt Times New Roman, Times, Serif"&gt;  In December 2012, NPAEP sold its rights to acquire the properties  of NPAMLP under the 2003 Agreement for $&lt;font style=" FONT-SIZE: 10pt"&gt;100&lt;/font&gt; in cash to EBL&amp;amp;S Realty, LLC.  (&amp;#8220;EBR&amp;#8221;), a company in which Lipkin owns a minority  interest. The majority interest in EBR is owned by employees of  EBL&amp;amp;S Property Management, Inc. and its affiliates. It is  contemplated that EBR or its affiliates, will acquire the  properties of NPAMLP effective as of December 31, 2013 pursuant to  the 2003 Agreement.&lt;/div&gt;  &lt;/div&gt;        </NonNumbericText><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell></Cells><ElementDataType>nonnum:textBlockItemType</ElementDataType><SimpleDataType>na</SimpleDataType><ElementDefenition>This element captures specific management and other agreements including any negotiated amendments.</ElementDefenition><ElementReferences>No definition available.</ElementReferences><IsTotalLabel>false</IsTotalLabel><UnitID>0</UnitID><Label>Future Interest Agreement</Label></Row></Rows><Footnotes /><IsEquityReport>false</IsEquityReport><ReportName>Future Interest Agreement</ReportName><MonetaryRoundingLevel>UnKnown</MonetaryRoundingLevel><SharesRoundingLevel>UnKnown</SharesRoundingLevel><PerShareRoundingLevel>UnKnown</PerShareRoundingLevel><ExchangeRateRoundingLevel>UnKnown</ExchangeRateRoundingLevel><HasCustomUnits>true</HasCustomUnits><IsEmbedReport>false</IsEmbedReport><IsMultiCurrency>false</IsMultiCurrency><ReportType>Sheet</ReportType><RoleURI>http://www.0000926843.com/role/FutureInterestAgreement</RoleURI><NumberOfCols>1</NumberOfCols><NumberOfRows>2</NumberOfRows></InstanceReport>
