Proving that a value has been extracted from a witness

Hey, I’m not new to zk proofs but never coded anything before. I have in mind to do some processing on encrypted data using FHE and return it to my user. I think I can figure out this part. However when they receive the encrypted message they need to send me back something they would extract from the decrypted message (which I would never see) and I need them to prove me that it’s been extracted from it. How could I achieve something like that ? Of course that would be extracted using a certain function they would run on it.