wrong prepatment amount

henrytsui

i am using nav 2009 r2.
I have the following problem please help :roll: :

1.create a sales oder using non-local currency (say item A,qty=100, total amount =$100 and prepayment 20% = $20)
2.post prepayment invoice($20),and "Prepmt. Line Amount Excl. VAT"="Prepmt. Amt. Inv. Excl. VAT" column both shows $20.
3.partial ship (qty=50) and invoice
*at this point,everything is fine.
4.after some day ,i want to ship the remain item ,then when i change the posting date, the system ask me
"DO you want to update the exchange rate?". After clicking Yes ,i find that the system recalculate the prepayment amount.
"Prepmt. Line Amount Excl. VAT"= 30,"Prepmt. Amt. Inv. Excl. VAT" = $20 and asking me to pay the full prepayment amount before posting the remaining item....

please ask me question if my problem is not clear.