Payment Instrument not set on Pending Contributions' payments when marked "Completed".

There's an inconsistency in how payments are recorded in CiviCRM depending on how you mark them completed.

Scenario 1

• Record a pending contribution for a contact with whatever payment instrument you like.
• Complete the payment by selecting More » Record Payment and entering the payment.
• Click the pencil to edit the resulting payment. Everything looks fine.

Scenario 2

• Record a pending contribution for a contact with whatever payment instrument you like.
• Complete the payment by selecting Edit and changing the contribution status to Completed.
• Click the pencil to edit the resulting payment. The payment instrument is blank. Moreover, any attempt to change the payment instrument results in a fatal error (similar to core#264 (closed)). See below.

The backtrace below seems like a symptom of the root cause, which is in the second step (changing the contribution status) not the third step.

CiviCRM_API3_Exception: "Mandatory key(s) missing from params array: payment_instrument_id"

#0 /example.org/sites/all/modules/civicrm/CRM/Financial/Form/PaymentEdit.php(213): civicrm_api3("FinancialTrxn", "create", (Array:9))
#1 /example.org/sites/all/modules/civicrm/CRM/Financial/Form/PaymentEdit.php(181): CRM_Financial_Form_PaymentEdit->submit((Array:5))
#2 /example.org/sites/all/modules/civicrm/CRM/Core/Form.php(489): CRM_Financial_Form_PaymentEdit->postProcess()
#3 /example.org/sites/all/modules/civicrm/CRM/Core/QuickForm/Action/Submit.php(74): CRM_Core_Form->mainProcess()
#4 /example.org/sites/all/modules/civicrm/packages/HTML/QuickForm/Controller.php(203): CRM_Core_QuickForm_Action_Submit->perform(Object(CRM_Financial_Form_PaymentEdit), "submit")
#5 /example.org/sites/all/modules/civicrm/packages/HTML/QuickForm/Page.php(103): HTML_QuickForm_Controller->handle(Object(CRM_Financial_Form_PaymentEdit), "submit")
#6 /example.org/sites/all/modules/civicrm/CRM/Core/Controller.php(351): HTML_QuickForm_Page->handle("submit")
#7 /example.org/sites/all/modules/civicrm/CRM/Utils/Wrapper.php(113): CRM_Core_Controller->run()
#8 /example.org/sites/all/modules/civicrm/CRM/Core/Invoke.php(283): CRM_Utils_Wrapper->run("CRM_Financial_Form_PaymentEdit", NULL, NULL)
#9 /example.org/sites/all/modules/civicrm/CRM/Core/Invoke.php(84): CRM_Core_Invoke::runItem((Array:14))
#10 /example.org/sites/all/modules/civicrm/CRM/Core/Invoke.php(52): CRM_Core_Invoke::_invoke((Array:3))
#11 /example.org/sites/all/modules/civicrm/drupal/civicrm.module(345): CRM_Core_Invoke::invoke((Array:3))
#12 [internal function](): civicrm_invoke("payment", "edit")
#13 /example.org/includes/menu.inc(350): call_user_func_array("civicrm_invoke", (Array:2))
#14 /example.org/index.php(22): menu_execute_active_handler()
#15 {main}