Breeding loans can be done successfully, but the most important in my opinion aside from the piece of paper is picking the right person to do a loan with.
If you have a contract which I recommend (also I never use one with the loans I have made) everything need to be in writing from who gets what, to who is responsible in the event the animal must go to a vet or even dies. (The more details the better)....how long of a at period, what happen if the female does not go that season, should the breeding stop and the animal be returned or should the animal stay another season.
It is also important to communicate during the loan, does not mean people should get an update each week, but updates on how the animals are doing whether or not they are eating, when they are building, ovy etc