If the town isn't there after the MC returns, then the king fucked up. The town is the MC's biggest asset at the moment, but whatever he gains through the war is his to keep. This includes any potential cities or fortresses he conquers. The only reason why the border town is important here is because the MC is fond of it. Destroying the town while he is away fighting the war would only needlessly aggravate the MC. Admittedly, it's pretty clear he's already setting up against the crown, but such an act would utterly eradicate any slight chance of amicable co-existence that remains.
I expected someone to say this, and you're even admitting yourself, hes already hostile.
The only reason to keep the town would be to get into a schemes war with MC to try and drain him dry using the town as a hostage.
Lets be honest, no way this king would have a shot at winning such a fight, letting MC go away as far as possible with as little contact as possible has to be better than such a confrontation.
Also if MC only has the conquered land, you could say that he's outside the range of clause 1, allowing the king to straight up attack him since hes outside the borders of the kingdom depending on how you spin it.
Well you could do the same with the town but that would be far less convincing.