The situation: A wizard is going up against a mook who has been tagged with the boost Off Balance thanks to the wizard's ally pushing him around. His roll is good enough to hit already -- if it wasn't, it'd be straightforward to use the free invoke to add +2 and hit. But his roll was under the amount of power put into the spell, which would cause Backlash. Can he use that invoke to boost the attack for that reason? I'm not sure about the narrative here: it seems like the power was too much for him to handle, and I'm not certain that his foe being easier to hit really negates that, but at the same time, it seems like this should be a tool he has in his toolbox for this kind of situation...?
The situation you've described sounds hard to justify within the context of the story: Backlash is usually a fairly personal thing and only aspects directly affecting the wizard are easy to justify in helping to control a spell. I can't say whether it's legit or not though; although it seems very far-fetched to me, only your group can really decide rule on the narrative justification unique to the particular situation. So I'll talk about the general case of justifying invokes and how I handle it.
Most of the time, it's pretty obvious how an aspect relates to the action it's being invoked for. When it's unclear, the GM (or any other player, really) should speak up and say they aren't sure how the aspect is useful in the situation. Usually it's up to the person using an invoke to justify the usefulness of the aspect to the task at hand, but often the rest of the group will be eager to give suggestions!
If the group can't reach a satisfying agreement on whether the aspect is relevant to the action, the GM is the final arbiter. We're encouraged to be generous about allowing invokes, especially if it permits awesomeness. (If the group runs into this sort of problem frequently there might be a mismatch in what kind of game each player imagines they're playing. We've got a lot of other questions about how to handle that if it comes up.)
Personally, I encourage players to clarify an aspect's usefulness by adding to their narration of the action: my rule of thumb is that you can invoke anything you want, so long as when the action's resolution is described each invoked aspect gets called out. The player's creativity while stretching to justify a far-fetched invoke often brings new, exciting elements into the conflict, so I'm always inclined to let them at least try.