What is the ruling regarding someone who vowed to slaughter a yellow cow upon his son's recovery, but died before fulfilling the vow, while now only black or red cows are available?

If the deceased did not fulfill his financial vow (such as sacrificing a cow), the value of the vow must be discharged from his estate, if available. It is permissible for his son to volunteer to fulfill the vow on his behalf. Sacrificing a cow of any color is sufficient, as specifying the color of the sacrificial animal in the vow is not binding, because there is no act of worship or obedience in it.