Allow partial payments when original invoice is in a locked financial period

When making monthly partial payments on an invoice originating in a locked period, the partner has to unlock the books to allow the partial payment to be cut by staff, then the partner has to re-lock the books. This is a recipe for disaster as the prior year books could be compromised with the opening and closing every month. Even if the payment isn't in a locked period, one must still open the financial period.

Would save everyone involved time and effort if we could remove this restriction.

Legacy never required the opening of a financial period to make partial payments.