Field help (BPCPYR1) 

The Pay-by BP is initialized by default by the pay-by BP associated with the sold-to customer in the latter's record in the case of a direct delivery. Otherwise, the pay-by BP of the original order is used.

It is possible to:   

SEEINFO Special features linked to the inter-company: in the case of an inter-company order automatically generated from a purchase order, the pay-by BP corresponds to the customer associated with the invoicing site entered in the purchase order. It cannot be modified in this context.