Can a character be roped into Infernal Temptation (per the imp familiar unique ability) if they are unaware they are dealing with a fiend, or that penalties might apply?
I would go with the 'fine print' idea. The detrimental part of the bargain has to be stated in order for the bargain to be valid. It doesn't have to be highlighted.
So the Imp doesn't have to double check that the character understands the downsides. They are allowed to downplay the consequences. They are allowed to mention it while the character is distracted by something else like combat (but not while unconscious or confused or deafened or otherwise unable to hear and understand). And, no, the fiend doesn't have to inform the character that they are making a deal with a fiend.
Also, make sure that this is all being done with the player's agreement. That one does need to be clearly stated and agreed to. Only trick the character.
Yeah, never trick the player with something like this.
Let the player choose whether their character would go for something like this.
Honestly, anyone that knew the consequences or knew they were dealing with a fiend probably would not accept the bargain. The benefit isn't that great for eternal damnation. Like one reroll on one attack or save. That's terrible.
If the benefit was 1 hour where you could roll twice and take the better on everything you do and then your soul is guaranteed to go to hell. Like maybe whatever I'm about to do is important enough to go to hell over.