Legal Question in Real Estate Law in California

No, because it was not true. Most likely you didn't pay the full amount of interest due, because interest acrues daily. If you want to pay off a large loan, you have to get a pay-off demand from the lender which will be include all interest and charges through a given date. If you pay by that date it is paid off. If not, then more interest will acrue and you need to get a new demand. By trying to pay it off without getting an interest calculation through the pay-off date you short paid and the loan was not paid off. You can't change that by writing something on the check, and you can't force the lender to choose between rejecting the payment or accepting a false statement you wrote on the check.

Writing "payment in full" or similar expressions on a check creates an enforceable "accord and satisfaction" of a debt under certain conditions. These are:

1. There must be a bona fide dispute as to the amount owed. If there is a written loan agreement or the like between you, signed by you, which clearly calls for the additional interest being demanded (for example, if you missed the last couple of monthly interest installments) there cannot be a bona fide dispute between you, and the check is ineffective to pay off the loan entirely.

2. The person accepting the check must not have indicated rejection of the concept of accepting the check as payment in full. All the person accepting the check has to do to knock our your "paid in full" is to cross it off, or perhaps write "NOT" in front of "paid in full." Because of this, you'll need to look at the check or a photo of it to see if the lender crossed off your words.

3. The "paid in full" notation is ineffective if the cashing of the check was inadvertent or without knowledge of the notation. Most people receiving $50,000 checks are going to take more than a casual glance at them, so this requirement is not likely to be a problem for you. You might look to see if it was endorsed in handwriting, suggesting that the payee handled it personally, vs. a rubber-stamp endorsement.

See Civil Code section 1526 for the applicable statute.