"Son, it is important that you finish your homework first, and then you can go buy the toy." "You need to finish your homework first, and after that we can go buy the toy for you." "If you finish your homework first, afterwards you may go and buy the toy." These are just a few different ways to say that that are proper, get the point across, and vary slightly in word usage. Do you need any further elaboration?

The closest word I can think of is "reciprocity." It basically means exchanging goods/services for mutual benefit. Although if you use this term, you would risk implying that your son is doing his homework "for your benefit," when the benefit is really his. There is also "give and take." That might be closer to what you're looking for.