OK, then presumptively, although he allowed you to use the credit card you would be liable to pay him for any charges made thereon.
This would be pursuant to both a verbal contractual agreement and/or the legal doctrine of "unjust enrichment".
A general equitable principle that no person should be allowed to profit at another's expense without making restitution for the reasonable value of any property, services, or other benefits that have been unfairly received and retained.
Although the unjust enrichment doctrine is sometimes referred to as a quasi-contractual remedy, unjust enrichment is not based on an express contract. Instead, litigants normally resort to the remedy of unjust enrichment when they have no written or verbal contract to support their claim for relief. In such instances litigants ask a court to find a contractual relationship that is implied in law, a fictitious relationship created by courts to do justice in a particular case.
Unjust enrichment has three elements:
First, the plaintiff must have provided the defendant with something of value while expecting compensation in return.
Second, the defendant must have acknowledged, accepted, and benefited from whatever the plaintiff provided.
Third, the plaintiff must show that it would be inequitable or unconscionable for the defendant to enjoy the benefit of the plaintiff's actions without paying for it. A court will closely examine the facts of each case before awarding this remedy and will deny claims for unjust enrichment that frustrate public policy or violate the law.
As to the first issue - a verbal contract - verbal agreements are valid and binding if one was made between you. It would appear that there must have been some agreement given your communication to him stating that you realize you owe the debt.
The last issue is - the statute of limitations.
A statute of limitations is a law which places a time limit on pursuing a legal remedy in relation to wrongful conduct. After the expiration of the statutory period, unless a legal exception applies, the injured person loses the right to file a lawsuit seeking money damages or other relief.
In FL - the statute of limitations for both breach of a verbal contract or unjust enrichment is 4 years from the date of the breach or when a payment should have been made but wasn't.
So under either theories of breach of a verbal agreement or unjust enrichment he does have a valid legal cause of action against you.
The only issue would be interest - was interest to be paid on the credit card charges?
The court might not award interest from the past - but they would give him a judgment against you which would automatically start accruing interest.
Now you do have a counterclaim against him that you should receive value or monies for - the return of the ring to him.
You can make argument that any monies you owe him should be reduced by the value of the ring (whatever you can show was the value of the ring).
But regretfully, yes, he has a strong legal case against you.
It's strengthed by the letter you wrote which would be considered an "admission" by you.
An admission is a voluntary
acknowledgment made by a party to a lawsuit or in a criminal prosecution that certain facts that are inconsistent with the party's claims in the controversy are true.
I am sorry that I don’t have better news, but please understand that I do have an ethical and professional obligation to provide customers with legally correct answers, even when an answer is not favorable to the customer.