According to to their stated motivation, they're not actually interested in the source code: they want to know how your software makes decisions, presumably imp

According to to their stated motivation, they're not actually interested in the source code: they want to know how your software makes decisions, presumably important ones on behalf of their business. The people who articulate such a request (management), as a rule, are not qualified to answer it by looking at the source code. If that is indeed their concern, they've probably filtered it through their internal development shop, who's come up with the idea that they could review the source code and answer. Management thought that was a splendid way to do it - no need to bother you guys with such tedious busywork (OK, probably slightly rose-tinted, but the general outline of the narrative is plausible). If this understanding is what they're really after, then that's what you need to think about answering. Worrying about ML as an opaque black box is a bit of a thing these days, so it will probably come up with future clients as well. If you answering this is not satisfying to them, and they keep insisting on the source code, and they can't articulate why, then they are not being honest, and you should walk away (or at least clearly state that if they don't withdraw that requirement, there will be no agreement).