My guess is the offer is significantly more than he is making here. I'm also guessing that if UK is willing to get anywhere near it he will stay. I don't think UK will have to match or beat it. He just wants to be paid what his market value is. Nearly every coach does this at some point or another. I don't blame him. I think he likes it here and would like to stay with Stoops. So, if it's a matter of getting a bump in pay, Mitch should step up.
With that being said, I don't think you can pay him more than the OC or DC are making, regardless of his recruiting skills.