Actually as always with RPGs the problem is nitpicking the rules and forgetting that this is a role playing game, not a boardgame. I’m a boardgame designer also and I’m all for modern RPG designs but if you forget to put the narrative first and before the rules you get to that kind of problems.
The player was trying to convince someone of something, by announcing the TN you actually made the position of the target clear: he is very contrary and can’t be easily convinced. At this point is entirely reasonable for the player to back down without rolling. Rolling would mean he thinks is idea is so much important that it is worth the risk of the roll.
Not rolling means that he concedes that his idea is not worth the risk and tells the target “OK OK you are right, let me go drink a beer”
Were is the problem?

On the other hand if he says he is attacking the orc and you announce the TN he can’t back up because he already committed to the attack.