These are the steps to see the problem:

1. I add a contribution with status "Pending" (contribution_status_id 2) and save it and close.
2. Then I open it and set it as "Completed" and save it.
3. At this point if I open it again and edit the payment details I am unable to save it: it just circles on forever.

Looking into the civicrm_financial_trxn table I find that the field "payment_instrument_id" is NULL. If I set it as one of the possible ids and repeat step 3 above it saves the record correctly.

Can anyone else verify? Is it a bug? I just have a plain install of civicrm 5.8.2 with drupal 7.61.