standard procedure for clearing is to use posting date exchange rate
it is technical possible to affect posting date (clearing date) manual with transactions F-44 F-32
but date of clearing will be same day as an invoice date
however
there is a logical question why you need this funcionality
if there are difference between invoice and payment it is excheange rate difference
an it is exact calculation no depending on date you choose to take ex rate
do you try to avoid exchange rate calculation and posting ?
cheers