Using Drupal 7.44 and CiviCRM 4.7.8.
When using CiviCRM 4.6.18 a Pending (Pay Later) contribution can be completed by editing the Membership that it relates to, checking the box 'Update Payment Status' and completing the payment. When this is done in a copy of the (upgraded) database in version 4.7.8 an error is displayed 'Please enter the financial type'.
This is almost certainly an error of the database upgrade or the application itself.