clearing of customer down payment , document still remain as open-item

Dear Sir,

We are facing following problem and details are as below :

a) On 01/01/2016 , an advance of $ 1000 received from customer . Currency rate on  the day was 68 INR / $

    Using Tcode FB05 , We posted advance receipt financial document .

    The posted document shown in INR is :

       Customer Advance               Cr      68000

        Bank Receipt                       Dr     68000

b) Subsequently on 25/01/2016 , an Invoice of $500 was raised . Currency rate on  the day was 69 INR / $

    The posted document shown in INR is :

         Trade Customer                  Dr     34500

          Sale of Mfd Goods             Cr     34500

c) On 25/01/2016 , We adjusted the advance of $ 500 against the Invoice . Tcode used is F-39

         The posted document shown in INR is   

               Customer Advance               Dr      34000

               Trade Customer                   Cr      34000

The problem is that  Invoice document is still getting shown as open-items , due to foreign exchange difference of 1 INR / $ .  

We look for a solution so that Foreign Exchange Gain /Loss of  INR 500 also get posted automatically during the advance clearing , so that the document do not remain as open-item .

The transaction which need to be further posted as a part of advance clearing is:

          Foreign Exchange Gain /Loss     Dr       500

          Trade Customer                        Cr      500

  I request expert to kindly help me   please .

With Regards

BS Mittal