100% approve. Would strongly consider rolling something like a d4 to determine how the demon reacted (modified by species, because a palrethees isn't likely to attack). So for a palrethees it'd be like:

1 - Furious, bitter anger. Every deal it makes for the next d12 weeks is entirely centered on ruining the fighter in the most humiliating and harmful ways possible. Leaving dead innocents (like children or loose acquaintances) in their rooms to frame them, assaulting loved ones repeatedly whenever they think they're safe to cause them suffering, slandering them ahead of their travels, or even just trying to assassinate them.

2 - Cold respect and a reward for the fighter's cleverness. Maybe money or even a magic item, that comes with huge strings. (It was stolen, its cursed, someone very dangerous wants it, etc)

3 - Indifference and pulsing its fear creating affect just to be spiteful and drive away the fighter

4 - Actual amusement and a decision to follow the fighter around. Their new "friend" would probably amuse themselves in the same way any demon would, so atrocities would follow the fighter indirectly, the demon would get to make deals with those the fighter wronged/conflicted with for their souls, and maybe the demon even "helps" the fighter on occasion. As seen through the guise of a demon's idea of helpful of course. It might also just wander away after a week or so after it got bored.

... aren't deals more of a devil thing? I thought demons were chaotic-evil and didn't necessarily care about honoring the letter of their deals

At first i was expecting the demon to demand them playing the best song in the world.

Reminds me when I played Star Wars: Edge of Empire and our group met Bib Fortuna. One of our group immediately lobbed a thermal grenade at him, killing him, which is not supposed to happen. Our DM then sent a fuckload of mercenaries after us because, well, we did kill Jabba the Hutt's second in command. The group was completely obliterated by the bounty hunters. I wasn't even mad. (Well, except at the idiot with the grenade.)

Ah but you see, the agreed exchange was wait 5 seconds for me, therefore since the demon waited for 5 seconds, the fighter must uphold the transaction and give himself to the demon

For a deal to be a deal, each side must offer something; the fighter offered nothing so it fails to meet the standards, I'd say. Clever attempt, but not quite.

But does the fighter have high enough int to do this?

What is this "reddit" thing marked at the bottom of the image? 🤔🤔