Prepaid vendor purchase orders

I am receiving invoices against a prepaid purchase order. The
material has been received, but when I pull up Receipt billing line
it shows me a variance for the entire amount of the invoice. The
variance will not allow me to post the invoice.

I posted the prepayment through the Add Advance Billing line in the AP
Invoice Entry and referenced the purchase order as instructed. There
is a debit to Prepaid purchase order and a Credit to AP in my GL.