Learned in Business Law 101 if one writes "Payment in Full" on check, it means just that. My husband was recently in a rehab facility to which my insurance and medicare did not pay the entire bill. I sent a payment of 5700 with "payment in full" in the memo section. Check was cashed. Now they are trying to squeeze my for an additional 4000 and cannot explain to my satisfaction. Please advise. Thank you.