You done great!! You both looked at the deer at the same time and both agreed that he, at that time for appearances sake, had not hit the deer and you took the fatal shot. Both left on agreeable terms and with a handshake to boot!!!!! Doesn't get any better.
I would have to take it a step above that you were considering even yet willing to give the deer back and sacrafice your tag to do so is comendable.
Question though, how did you know it was his truck that you came across later for you said he must have came in behind you without you knowing, did you know him before this event?