From a roleplaying standpoint, this is a terrible idea. It will actually make a warlock an unplayable character class. Just think about it, Devils and Demons which are very hard to summon into the world, found a way to cheat the system by granting power to mortals, they will be able to open a fate and enter unfettered? 2 things will happen, one, warlocks would be put to death as soon as they are discovered, and two, the world would be conquered by one of the demon lords thst entered the world and summoned its army.In our game "Warlocks" have glowing brands on their body that represent the pacts they have made. Each brand is one inch square per "spell slot". Slot one brand is one inch by one inch, slot three is three inches by three inches. Each time a brand is used there is a cumulative chance that the being granting the power will be able to enter the "warlocks" plane of existence and have "fun".
This is our representation of the "cost" of making a deal. How would your players feel if the warlock having cast a spell granted by a balor several times suddenly had the balor appear somewhere nearby?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.