"Well... since my Master is a devil... I doubt El General is going to take an affidavit from him very seriously...
At best he'll think we're trying to double cross him somehow, and balors are contrary enough to not go along with whatever we propose out of spite just because it is from a devil...
I can do this for you, if you insist, but I don't think it is a very good idea.
If you could get one from someone else, though... maybe a demon lord, if you can find out which one if any he was allied with... Or a commanding officer of his, if any exist in this time frame."