> there is so much to go wrong and "contracts" drawn up between people have been proved not to have any legal standing in lots of case
That's the truth for sure. I sold two puppies to a fellow breeder, with a contract saying part of the sale of the dog was to include one free stud from him on one of my bitches (specificied which one). As it turns out, by the time the dog was just over a year old, he and his sister were sent abroad to another breeder. How often have I kicked myself for not endorsing them 'not for export' (which was available then if not now?) but being a fellow breeder, and one I trusted (hah) they went with no endorsements. And obviously being out of the country any possibility of being able to take the stud I wanted went out the window. I did have his litter brother, but I didn't think he wasn't as 'right' for my bitch as the one I sold was.
Lesson learnt and part of why I prefer things to be 'outright' rather than getting involved in anything to do with contracts. And I'd never again sell to a fellow breeder either. Had that dog gone into a pet home I could have used him - and as for the brother I kept, I'd probably have homed him into a pet home, with access, and kept the sister I sold. Hindsight.
Just to add that if you know the people involved in this idea, and they have been friends for a long time (but as suggested, friendships can be broken over things like this!) you might be lucky but as it appears you don't have such a person/people, I'd sell these dogs and hope the new owners will show them. Far cleaner.