What is the ruling on one who vowed to sacrifice a buffalo, but instead bought a calf with its value, and is this permissible?

A vow must be fulfilled in the manner in which it was made, and it is not permissible to substitute it unless it is for something better. If a calf from cattle is better than a calf from buffalo, then it is sufficient; otherwise, it is not sufficient unless it is impossible to obtain what was vowed. In that case, its equivalent from cattle, camels, or seven heads of sheep would suffice.