How to enter exchange rate when manual receipt?

Hi,

   The company domestic currency is HKD. There're two or more USD invoices with different exchange rate to HKD.

   We receive HKD and apply it to USD invoices with another different exchange rate which is determined by our customer and not setup in P0015A.

   How can I enter this exchange rate?

   For example:

   invoice RI-1  $100 USD  exchange rate: 7

   invoice RI-2  $200  USD  exchange rate:7.5

   Receipt: HKD $1600 exchange rate: 8 (this is detemined by the customer)

   After applied the invoices, the open amount of two invoices should be decrease $200 USD(RI-1 open amount is $0 USD,RI-2 open amount is $100 USD).