Salvaged from oblivion on the gnucash-user mailing list [1]...

From David Beattie <dvdbeattie@yahoo.com> in message

<quote>
Currently, the Assign Payment feature can be run twice in a row, and the second time you run it, the stuff you've already assigned is not there, and there's also no indication that the payment is fully assigned already (the amount is still there in full).  I'm scared to find out what would happen to the original payment assignment if I did in fact try to assign it to another, unpaid invoice, and I don't want to corrupt my live data to risk finding out.  But I would suggest that populating that dialog with the UNION of the currently-assigned invoice splits and the available, unpaid invoice amounts, would be more user-friendly than what it currently does.  Then if a payment got mis-assigned, the assignment could be edited in an obvious way.  Doing the re-assignment could even change the amount sent to an invoice that the payment was already partly assigned to, in which case, the new amount could be combined in with the existing lot link, rather than adding a second split to the same invoice in the lot link.  Thus the "assign payment" would essentially be an edit button for the lot link, rather than only designed to create them.  Also, the "Assign Payment" menu item could be removed from Invoice lines (except credit notes) in the register, since it doesn't make sense there.  AND, it doesn't make sense to assign the payment to itself, so if assigning an unassigned payment, the payment itself should be deleted from the assignment dialog (it is visible there, currently, listed as a pre-payment).
</quote>

Note that the reference to lot links is not very relevant here. It has been rewritten since the message was sent to gnucash user. The core of the request is still valid though.

This request is closely related to bug 734865.

[1] http://lists.gnucash.org/pipermail/gnucash-user/2014-January/052260.html