Lets say you post a customer invoice with financial dimension value AXX, and then you draw the bill of exchange and post it. The system checks the master data related to the customer (it works the same with vendors), and changes the financial dimension of the original transaction with the one that the customer has, lets say AZZ. This way you lost trace, the system should keep the financial dimensions of the original transaction, and only take the dimension values of the master data if the original transactions don´t have values.