Hi
The best way to deal with this is to delete the spend money transaction and redo it correctly as a receive money against the bill.
In fact if the spend money is on the locked period it will not be able to be deleted.The bill if open will not itself be affected by the locked period
So you can unlock the period , delete the spend money and redo as a pay bill (making sure the same dates are used and then relock the period Reconcile the bank.
OR if you do not wish to unlock the period I suggest that you do the reverse of the spend transaction
So you receive money against the same account code as the spend money as at todays date same tax code etc
( this offsets the original incorrect entry), then you pay the bill as normal using the same date .
You have created a deposit and a withdrawal on the same day correcting the original error and paying off the bill, reconcile manually in bank and all should be good
Hope that helps
Lisa