Credit memos in SAP are used to correct a vendor invoice. Once the invoice and the credit memo difference is posted in SAP, any open items left in the invoice, credit memo and payment can be cleared. To take an example, suppose you purchase office supplies worth $500. A user posts an invoice received from the vendor. Now suppose the user comes to know that $150 has already been paid on the invoice. A credit memo is used in such cases. In our example, the user will process a credit memo worth $150 in SAP. The difference between the invoice amount and credit memo i.e. $100 would be paid to the vendor via check.