Difference not generated in payment

Can anyone help Please;

Sales Invoice amount is 10000 USD + 1500 USD for Tax, hence total Sales Invoice amount is 11500 USD

Rate is at 36 USD , I,e 1 USD =36 MUR on 01 April 2020

Sales Invoice in MUR is 414,000 MUR

Upon processing payment and the Exchange rate is changed to 45 USD either manually or through creating Currency Exchange list setup, the Set Exchange Gain/Loss button does not appear to post the difference generated. how to post the difference generated.
Same Currency USD has been used on both Sales Invoice and Payment