Having thought about this a bit, I feel like if I had turned a cold shoulder to his proposal to work for us in the future, he would have been discouraged about it. I could also have communicated in some other way that we work by the book.
At least then, we would have had the chance of working with other (possibly non corrupt) people in the organization to get our project through. I mean there are other people there who are enthusiastic about it.
Anyway, that's just me ranting. Thank you for your words.
Alternatively, you could have gone the comedic route by making the whole thing into a big joke, but it's not like things would have been better.