I am going to tell my story on how this has worked for me with a situation similar that im in.
Had a buddy outright buy snake, charged him a little more than the deal I actually got just for the fact that half of the price I got the animal from was because I have spent several thousand dollars with him in a year (like receive one snake and drop him a check in the mail to down pay on something I hadn't decided on yet) well the whole plan was he would take the snake, buy his food from me and at breeding season, I would then take the snake back and breed it and upon babies being hatched we would split the babies, keep what we wanted and sell the remaining, splitting everything down the middle. Well six months into it, I have fed the snake, taken care of the snake and still haven't came to an agreement on which one of my animals to breed her to, not the best deal in the world but getting into an arguement of what we could/should do isn't worth the hassle.
Contracts are definately a great idea, but the very least prepare yourself to lose in the whole situation.
Just my thoughts on the whole deal.