I’ve categorised this as a bug, but it is half bug/half feature. I am aware there are ways around this.
I have a payment from a client which is made up of a payment for additional charges from last months invoice and an over payment made in error. Rather than credit the customer I want add the balance to the client account to use for next months payment. It would appear that this is not achievable from the bank tagging screen as attempting to select an invoice and the assign to account option is not possible.