The Debit balance recovery is usually made by raising a credit memo for the regular vendors. However if there are no future transactions from the supplier, we ask the vendor to send the check / make an EFT for the amount due from him.
For material/Goods Purchases, the Po is required. In the Po, QTY, Product Name, Where is delivery price, Terms and Conditions. In the po there is no matter of final amount. It is just the approximate amount.