This sounds a bit like a corner case.

The steps to reproduce are:

• Create a payment and leave the invoice empty

• The user must navigate to the Allocate tab and create one or more records there

• And then, the user must come back to header tab and fill the invoice field - which I would not really expect from a user that already went to detail tab to fill the invoice(s)

• Note that if the invoice field is filled on Payment, then records cannot be saved on Allocate tab, also it would be very easy to simply hide the Allocate tab when invoice is filled - simple read only logic on the tab

The patch suggested creates a virtual column, which are not recommended for performance issues, so, there is no damage here, closing as Invalid.

Lowering priority as the payment cannot be completed

Just make virtual column in table payment, is count line of allocate.
make logic read-only of invoice, order, charge by this column