I attempted to do this and IT DID NOT WORK!!!!!
I thought I was dealing with someone that I trusted. I had bred to her mares before. I admired one of her mares (Mare #1) and offered to breed to another mare (Mare #2) in exchange for getting a foal out of the one I liked. So I breed both mares and both get in foal. When the owner comes to pick up the mares, she tells me a sob story of how she can't afford to feed both mares and since the foal is going to be mine anyway, she thinks I should keep Mare #1 at my place and foal her out the next spring. I reluctantly agree fearing that if I don't, she won't take care of Mare #1 who will then lose her foal. I tell her I won't be responsible if something happens to Mare #1 while in my care and won't be liable for any extraordinary vet expenses. She agrees and says she will assume all liability. Well, you can guess what happened. Two weeks prior to foaling a stray dog scares the mare (does not attack her, she was safe behind good fencing) and sends her into premature labor. She has a dystocia and I call the vet. Vet can't get the foal turned and we have to do a Caesarian to save the mare. A fetotomy was considered too risky for the mare. I call the owner and tell her the situation and ask if she wants the operation done. She says yes, please do it. The mares value is well below the cost of the surgery. The mare gets through the surgery great, but the foal is dead (of course). The mare stays at the vet for two weeks and then needs to stay at my place for additional two weeks before she is safe to travel. My vet sends the bill to the owner and never hears from her, but I don't know this yet. After the month post surgery is over. I tell the owner to come and get her, giving her a specific time in the afternoon because I won't be home in the morning. I also tell her that I'll have the vet bill for her (just got told by the vet there was no payment yet). You guessed it...the owner came when I wasn't home and took the mare. She then never paid the bill, ignored certified letters, phone calls, emails, etc. I paid the vet bill because I need to maintain a good relationship with him. The end result is that my stallion serviced 2 mares and I paid out $1800 in vet bills and all of that mare's care for the previous year. Oh, Mare #2 had a beautiful filly just before Mare #1 lost hers. I did make sure that AMHA knows that I won't sign the stallion certificate, my only recourse.