If I got it right: Player character A has a demon pact for power, evil npc B wants to use the party for a quest, evil npc B has a demon pact with a demon that is the boss of character A's demon, you want character A to not shortcut the quest and not zorch evil npc B. Optional - character A may be required at some point to back evil npc B against the party as a result of the pact.

This seems mostly an issue with how its sold to the players, as all the fiction bits (as I understand them) seem to line up nicely. Now, personally I tend to run more "9-to-5 working day" type demons who map more to dysfunctional office workers than to "evil for the lulz" type demons. At least for most of the organized pact maker demons. So this would offer me an opportunity to show some of the office drama & corporate hierarchy in demon society to the players, who might then be able to leverage a bit of it for themselves.

So I'd open with character A's demon saying something like "Oh, him. Yeah, orders from corporate, we gotta help this one... Oh he's got a line from my bosses' boss, so its standing orders that we have to help. It sucks but thats what it is." Then eventually let on that if they can pull of npc B getting a partisl success or failing after a certain point, while still "helping" npc B, that there would be some vague political advantage in demon-land for the faction of chatacter A's demon.