How do you enforce Application BU = GL BU in for example Requisition entry

We have multiple Business units - GL, AP, PO, etc etc. Due to certain user default configurations there are default business units setup for a user. A user may have access to multiple business units. In Requisition entry, for example, a users' default BU = BU_01. When entering a requisition, the application BU(header BU) = BU_01, but they enter GL BU on the req line = BU_02. This eventually creates interunit entries in AP. If they changed the Header BU = BU_02 then no interunit entries would be generated. Is there a way to ensure\enforce that the header BU and