I'd say you have four legs to stand on. Providing reward for your service was agreed upon before you took the horses into your care, you have every right to keep the animals until full settlement of the account has been made.
If there has been no communication as was expected there is also some law about abandonment of an animal. Not sure but this may enable you to keep both horses.
I can't imagine how hard it would be to give up an animal after three years of care.