I think I would negotiate a contractually obliged introducer/finders/referral fee (and which party is to pay it) prior to making the introduction. After the event, what do you think you can get away with? Depending on the level of involvement you have had, and what the level of facilitation (initial and ongoing) is, would determine what might be reasonable. If you are doing nothing, and the project/work has no ongoing dependence on your involvement or compliance, then it would seem to me that you are unlikely to retrieve the value that you might have been able to ascribe to a contractual agreement prior to introduction.
There seems little point suggesting that, say, 5% might be appropriate, if the margin for the work is, say, 3%, and an additional cost of your fee has not been factored into the offer.
After the event the question would seem to be, 'how much of a cut are you prepared to give me, to retain my goodwill?'. I'm not sure that anyone can answer that better than you.
You might also want to check the tax position of that (is it VAT'able, where does that obligation lie).