Sounds like this guy was taking the proverbial. Either he should have invited you to replace it at the time, with no penalty, or he should have confirmed he conceded the hole. Anything else over an 8 inch putt is gamesmanship.
'That's good' is a well accepted, if somewhat American, term for concession, so it is his problem if he said something that could be reasonably interpreted, in the circumstances of an 8 inch putt, as concession but didn't mean it. A concession, once given, cannot be withdrawn. If you had hit a 3 wood to 20 feet on a long par 4 or a par-5 and he said 'That's good', that would not reasonably be interpreted as a concession.
