Financial dimensions on lines for Free Text Invoices are coming from the master configuration (Customer, item, procurement category, employee, etc.). Any Configuration on Main account overrides all dimensions coming from above at the time of posting.

However, the user does not expect this type of behavior. Whenever user will try to enter a main account in Free tax invoice, they will look for the fixed dimension value mapped with main accounts to show up under Line Details> Financial Dimension.

So it may make them feel hesitated to post the free text invoice.