If I were you and I decided that this is worth it, I would try to arrange the deal so that it only occurs after all of their other due diligence has been comple

If I were you and I decided that this is worth it, I would try to arrange the deal so that it only occurs after all of their other due diligence has been completed, and the source code audit is the last remaining obstacle to doing the deal. And I would insist that they first define, at least roughly, what it is acceptable and unacceptable. So, after the audit is completed, unless they can point to something in the code that is unacceptable, then they are compelled to go through with the deal. You want to avoid a situation where you accommodate them on this, and they come up with some other hoop you have to jump through. Or where they give you some vague excuse like, "thanks, but we decided to go in a different direction" and walk away.