So I know there is no way to undue an invoice marked as paid when it wasn't, but how can I convince the recipient that it was in fact not paid. They are arguing with me about it.
Hey @Cbenoist1! Welcome to the Community.
You could show the recipient your Transaction history or Deposit history and show there's no deposit in there for the payment they owe.
You can also advise they check their bank statement to see it isn't on there if they haven't paid it yet.