How to pay an invoice in the "Payment processing" status

When a payment was initiated on one or several documents but did not go through (or was abandoned by the user), the document(s) get the following status: Payment processing.

To pay a document in that status, the steps are the same as a normal payment with an extra step to dismiss the warning that a payment has already been initiated for the document(s):

1. Select the document(s).
2. Click on Pay documents.
3. A warning message is displayed to let you know that a payment has been started for this document. 
   To continue, you'll first have to click on Dismiss.
4. Choose your payment method then click on Pay documents.