I vowed a sacrifice, and it died. Do I owe anything?

If a person designates an animal and vows to slaughter it, then it dies without any negligence on his part, he is not obligated to replace it, but he must offer an expiation for a broken oath (kaffarat yamin). However, if someone vows to slaughter a sheep without specifying it, then later buys or designates a sheep to fulfill his vow, and that sheep dies, he is obligated to replace it to fulfill his vow.