When browsing for new ponies or bloodstock, it’s normal to want the best you can get, in breeding terms: a foal out of a platinum mare. Unfortunately, you’ll be hard pressed to find owners who will part with these foals for a sum of money that won’t make even the richer stables think twice. Heck, you’ll be hard pressed to find anyone who will part with these foals for a sum of money. So, it’s time to throw in the towel and go home, right?
Sort of.
Let’s say Farm 1 has foal out of a platinum ranked Mare A. You want foal out of Mare A. A lot. Even if you don’t have that much money, you might still have a chance of getting this foal. Owners are much more receptive to swapping horses than they are to flat out selling horses. Rather than simply losing a potential broodmare/stud prospect of which they have many of its relatives, they get a new broodmare/stud prospect AND diversify their lines. You, too, would be doing the same. It’s a win-win situation.
However, in order for a swap to be accepted, usually, you need to be offering a foal of similar quality. This means, if you’re asking for a foal out of a platinum mare, you better be offering a foal out of a platinum or an-almost-platinum ranked mare. Getting a foal out of Mare A requires sacrifices on your part, too. This isn’t a hard and fast rule, but it’s a darn good rule of thumb. If you only have lower ranking mares or unproven mares, it’s still worth a shot, particularly if the unproven mares are former stakes winners. However, just like when offering money for a horse, you don’t want to insult the owner (e.g. offer $10,000 for a stakes winner). It is not OK to offer the foals out of a bronze mare or from an unproven mare that never broke her maiden.
Congratulations! Mare A’s owner has agreed in exchange for a foal out of your platinum ranked mare, Mare B. So, in addition to swapping among existing foals that you own, you can also swap for next year’s foal, allowing Mare A’s owner to pick Mare B’s date and vice versa. The following year, you swap weanlings.
Of course, swapping foals you take a risk on the foal you gave up being a super star, and the one you receive being a dud. But that’s part of the game. And, even if that’s the case, you still have the broodmare.
Happy swapping. 
. When I lease a mare, I’m basically at the mercy of the leasee to remember to return her once she foals because I forget about the mare by the time a year’s up.