Get a lawyer and an expensive haircut or you're begging to have your scraggy stolen from right under your nose. Document every interraction you have. Make sure you look good mostly naked.
People who buy music are in the business of comodities. They're looking to sell anything they can, and so they overbuy. If someone buys something from you, they own it. If you wish to retain any right whatever over your creative work you will need some weight to throw around. (see above paragraph)