Balard is basically correct, but if you looking for guidelines based on the power level of the devil, I would suggest something like:
Lower level devils - can't grant actual "Wishes". They will make bargains, but they can't grant anything beyond their own abilities, or beyond the abilities of whatever entities owe them a favor or service from previouis bargains.
Very clever devils can manipulate people and events to bring about the results they want, in order to meet their part of the bargain, although often not in a way that the PC would like. That's the problem with dealing with the devil, the "letter of the law" over the "spirit of the law". "You wanted the children out of the orphanage right? Well now that it's a pile of cinders, no more children will be forced to grow up there. The few who died were innocents and will go to a better afterlife. "
Very powerful Devils may be able to grant actual wishes, but those would be few and far between, and they would always be looking for loopholes in the wording of the wish so that they can corrupt the intent.