If you owe a client pay them back

Since the 21st Sept the tedious back and forth correspondence with Ampath started. Emails regarding money Ampath owes me. They don't send everything they need from me to be paid back at once. All is done in drips and drabs. I have sent proof of payment directly from the bank of a payment they were questioning before they can now pay me back the amount owing. Proof of payment was sent to the given *** more than once. They are asking for proof of payment still. More than this I can't do. It seems like they are stalling and hoping that I'll tell them to just keep the money they owe me because why is everything so long winded?