You have two options, you can fight this by asking for verification of the debt from the collections agency, as well as notifying the credit agencies and providing your own proof (which you don’t seem to have?). Or you can pay it and hope the damage is not enough to impact your home purchase. If your priority is to buy a house and you don’t have proof of cancellation, your best bet is to pay it off and move on. You stand to lose more if it’s temporarily removed from your credit while you fight is and then added back right before closing on a home, ranking your home purchase.