So in that case, you have a reason to be disgruntled somewhat, especially since the time you had to spend on that delayed your work on other in-house projects, which meant you had to work late, taking personal time away from your family (if you have one).
How you handle this is up to you. It sounds as if this were a unique, one-time thing, so my general advice to you is to still be proud of the job you did such that the person wanted to pay, and consider it an act of kindness/generosity toward your boss, hoping it will earn you favor with him. If this turns into something that he asks for again, and you don't want to extend yourself out for him, then you should mention your workload, and that doing work for someone else will delay your other responsibilities. At that point, be VERY attentive to your bosses reaction. Don't consider the matter close yet, but try to judge if he thinks that you should do it anyway. If you can at that point, continue explaining that the last one-off job you did put you behind such that you had to work late hours to catch up, and you would prefer not to put yourself into that kind of time crunch again. Ask him if he minds that doing the one-off project will delay your normal responsbilities, and again be very attentive to his response. If he's understanding, then problem solved. Let him suggest a solution that you can both agree on. If he seems to think that it's your problem and not his, then you'll have to decide what to do from there. Suggest to him that you could do the work on a contract basis directly for the company, negotiating your own pay with them, and see how he feels about you moonlighting. If he's a reasonable person at all, he'll understand your position and should accommodate you. If he's not a reasonable person, then you'll have your answer and know that you should really seek employment some other place where your time and effort is respected.