pay in full does nothing

[weex]

If you have a posted sales order in AR, select it and click 'pay in full' from gear menu. It'll say 0 of 1 records paid in full.

[weex]

Turns out it will change the amount paid if a payment type is already selected on the order and it hasn't been posted as an invoice. Perhaps this is just an issue of lack of documentation and lack of an error message.