BU does not default in Payment Worksheet if the customer doesn't have item activity (from the past)

Hi,

Hoping to get some clarifications on the following issue:

If a customer is newly created (recently) and so far doesn't have any item activity, whenever I try to build a payment worksheet BU doesn't not default and it's not either available for manual selection. Scenario: a payment has been received and the customer is identified. However, for the moment there is no invoice to apply so user needs to put it on account in order to generate accounting entries and post to GL (for us, deposits are created in AR via LockBox flat file and further processes). In this case system does not default or make available the BU