I added some new info in my comment, but I think the edit feature won't work on all nostr clients so here it is:

The idea is to have a database of verified checksums for Cashu Wallets, where these wallets have implemented code to ensure that users cannot have a copy of the token where they can spend it before the intended person do. This database could even be hosted on Nostr nodes, allowing the community to provide 👍👎 feedback on wallet checksum builds. Any unknown wallet, or known wallet with altered code resulting in a new checksum, would trigger a warning to the recipient of the eCash token.

The challenge now is how to implement this system in a way that allows different wallets to establish legitimate consensus during offline transactions, as I still see the possibility of malicious actors finding ways to tamper with it and circumvent the protections.

The idea is good, but when users have the power to reclaim an eCash token that they were supposed to give to another person as payment, and leave the other person with nothing is kinda annoying, in the end, you should only trust receiving eCash from people you really trust, else it would just be a race of "who can reclaim this token faster". If we are trying to mimic physical cash bills, we have to take into consideration that once you hand over a $ bill to someone, it not in your possession anymore.