
There seems to be 2 or 3 ways that a few people use.
1) The loan of a bitch agreement using the KC form. This isn't probably the best method if selling a puppy to get a pup back from though.
2 ) Selling the pup at a pet price BUT endorsed with a written contract saying that the puppy was sold as a pet and not for breeding. A little papragraph about If in the future a litter is agreed the breeder will get a pup back from the first litter. Endorsements will remain etc etc. ( there is huge loop holes here legally because by having a sales contract you have sold the bitch, it is however it is used quite often so trust is a big thing )
Another way to do it is to have a "keepers" contract done so that the bitch hasn't been sold, and will remain in your name for everything till you have a puppy back. Never use the word sold in the contract. The person will be the keeper until a litter is born.
I think if you want to, you have to appriciate from day one there is a risk. You have to hopefully really trust the person. If things go wrong you have to be able to call it a day also. Some people go to crazy lengths and it becomes about them and not the dog sadly.
My experience won't be of much help even though I have done it myself . I was the buyer not the breeder . To be honest all work fine for me . We both knew each other well, both had legal knowledge so know really where we both stood to the point we actually didn't do any contracts. ( I am not suggesting or recommending people do this though) I made the verbal agreement and stuck by it 100%.