In invoicing, the repayment method is determined for the bill credit that was established when the bill sum total was determined. For this purpose, you must maintain the repayment method string in Customizing. Choose SAP Utilities   ® Invoicing   ® Invoice Processing. In conjunction with the repayment method control in the contract account, invoicing checks the payment methods specified in the sequence. The system uses the first repayment method that meets all the criteria specified in Customizing.

Alternatively to the standard repayment method determination, you can store a separate logic using event Payment Determination for Business Partner Items Bill (R430).