I am developing and selling a piece of software over the internet. Recently a customer has asked me for a copy of the source code, so he can add custom features and maybe also assist to fix bugs. He also offered I could use the bug fixes in my main version so other users would benefit as well.
Now the question arises, under which software license I can disclose my code, without losing ownership over it. I want to keep the right to sell it, even if other people can volunteer to maintain it.
I have looked at some public license models, but all of them seem to either grant everyone permission to sell my software, or to make it completely free to use.
I don’t want to be in a situation, in which one of the contributors suddenly changes his mind and I am forced to remove all the code he added. Or worse, allow him to sell the entire software on his own.