Tas: how to allocate an invoice to a pre-payment?

Hi I dont know if i am in the correct area but i am tearing my hair out trying to work out something in TAS.

Would anybody know how to allocate an invoice to a pre-payment made?

Ed

Go in as if you are making a payment in the purchase section ( i presume it is a purchase invoice), except ensure that where the bank a/c details comes up, use the arrow beside it to point it to the blank bank account, hit the tab button, the payment will come up, double click on it, and it will bring you into the invoice, and double clic on it.