I don't know how copyright laws work in Russia, but it's a bit of a gray area in America when you write code for someone unless it's explicitly spelled out in the contract. The end product (compiled binary or whatever they're paying for to "just work") belongs to them, but the source code does not belong to them unless it's explicitly stated in a work-for-hire contract that all of your intellectual property goes to them while you're hired. I always make it clear in my contracts that I am not re-selling rights to anything I build that's derived from FOSS or, even my own code if it's part of my own open source projects; the client cannot own something that is not for sale, just by hiring me on an hourly basis. I make sure to communicate that with them verbally as well beforehand, in case they mistakenly have the idea that they are buying the rights to something that I can't legally sell them. Usually I present them a list of modules and files that will be used in their project which (a) are FOSS, (b) belong to me, and (c) which I will write for them that they can reuse/rewrite/resell the source code for.
