Ask them to produce proof that they were not paid, since you have a receipt that you did pay. It is not your responsibility to ensure "the payment went through", after getting a receipt. On the receipt there should be a credit card number (or last 4 digits) that you can use to identify the account. If that statement shows the payment, send both the receipt and the statement to the hospital as proof. If they continue to cause problems, inform your State's Department of Health and Human Services that the hospital inappropriately sent your bill to a debt collector despite receiving payment. Also inform the hospital's billing department about this, and that you will not be paying. That may get them off their butt and figure out on their end what happened.