Is that canon? Sounds like one of those ultra homebrew D&D things to me - where the DM has more to say than even Lord AO himself/itself.
Talking about this guy:
The Single Strongest God Of All Time In D&DMeanwhile in a Bunker spacestation
orbiting our planet...
That kinda corelate with as i understanded it ...
I just wonder if your fiend patron shouldnt see you to be like "You wanted to screw me? No Eldrich Blast for you anymore, sucker!"
Hm~ some of the spells that Warlocks can use, can also be used by Wizards.
I wonder if once the Mortal has unlocked the inner understanding and intuition for a form of spell, this spell might be forever unleashable as long as the Mortal still has a spell slot left?
Devils getting angry about a Mortal trying to wriggle himself out of paying the due?

Differences are open to the individual I guess but I expect a veteran Devil more to be amused and maybe even "charmed" by the attempt of his/her Mortal trying to cheat.
They are high representatives of lies and trickery after all.
A Devil being tricked by a Mortal is probably a laughingstock at least for some while.
" Oh look that Devil over there was screwed over by his Mortal in a pact. "
" Really? Woooow what a lo~ser! Ha ha ha! "The smart Devil should anticipate and expect a possible betrayal. It is the Devils own responsibility to make his/her pact foolproof I think. Without an easy way out.