Unfortunately, you were not the owner of Bustabit when the 2 BTC was paid for the licence but are having to deal with something from the legacy. It is very good to see you are continuing a high level of customer services in the same manner that the previous owner/operator did.
Keeping that aside, I have some reservations about the claim being made about the licence. Regardless of the background issues he states he is going through, it seems the claim is being made because he wants to re-sell the licence back and receive the same payment (in BTC) that it was purchased for. Failing that, the secondary plan to validate the licence is the next best thing. By not having a written contract clearly stating the definitions, it was probably going to cause issues later one day.
It will be interesting to see the next update.
I have spoken to a lawyer, and I've been told that legally I have absolutely nothing, and the original writer of the code has legal rights to require the discontinuation of the code at any time.
Could you please have your lawyer contact me on
leo@bustabit.com and reference the details of the sold license? I will happily, free of any additional charges, work with your lawyer to provide all the guarantees you or your broker require (i.e. signing the necessary contracts, permissions, licenses, etc.).