incorrect financial_trxn when two changes made at once

Create a pay later donation with payment instrument of check. Imagine the check is not sent in and they call and get a credit card number instead which they process manually on the POS terminal. They enter this in CiviCRM by Editting the contribution, changing status from pending to completed, and change the payment instrument from Check to Credit card.

This results in 3 financial_trxn:
a) from=null, to=7 (AR)
b) from=7, to=12 (payment processor)
c) from=6 (deposit bank account), to=12

The proper way to handle this is not to create record c) since we know we are transitioning from AR to paid, and therefore we should ignore any value payment instrument had while status was pending.

(dgg / kurund: If despite this advice it is necessary to create a third transaction to keep the code simpler, then the transactions must be b) from=7, to=6, c) from=6, to=12.)