The yearling was paid for three months ago, she is theirs. If you offered them another filly at the same price, and they did not take it, KNOWING about this horse, then you have attempted to make it right and they said no. They were AWARE of the problem!! Now they are changing their mind?!!
The mare that was offered was not known to be bred at the time and was being offered as a not confirmed in foal mare- a single sale. It is now months later and they have decided to do something else? It does not work that way. What I offered someone months ago, and what is happening now, are two different things. You had already made your offer and it was refused.
If you insist on trying to make them happy, I would offer them another filly of equal value, or to apply the purchase price of the yearling they bought towards another horse for sale, but if the horse is now priced at more money, they need to pay the difference. It's a farm credit situation. I too would wonder about sending the filly there if they really dont want her, but do feel they had a chance to trade or exchange her and they refused.
On another note, I find it odd that at this stage, as a yearling, the filly's legs are not quite right. Are you SURE your farrier is trimming her correctly? If she was correct all along, why is she suddenly going off.. that sounds like an imbalanced trim as a Mini at this age has much of their growth done already.
Why didnt they take the yearling when they purchased her? Were they waiting for the colt to be ready also? They should have been paying board, etc.. on the filly in the meantime as she is weaned and ready to go and was when they bought her. She was already paid for in full 3 months ago and is theirs.
It is a tough call and I guess you will have to do what is right in your heart, but you already made and offer to exchange her and they were still happy to take her at that point. I would not refund any money- I would offer a farm credit towards something else. I know a LOT of farms who do not issue money back, only a farm credit.
This was not fair to you, to think the filly was sold, and the foal too, board the filly and get all her work done, etc... offer an exchange that they refused and now months later at the last minute they want money back.