Is it permissible to replace a sheep that was vowed to Allah Almighty—and which is available and one is able to slaughter—with another that is better, or by giving money exceeding its value?

If someone vows to sacrifice a specific sheep, it is permissible for him to sacrifice a better sheep, or to sacrifice a cow or a camel, because changing a vow to something better is permissible, whether the vow was specific or general. This is the view of the Hanafis and Hanbalis, and Ibn Taymiyyah chose it. This is indicated by the Hadith (definition) of Jabir, may Allah be pleased with him, that a man vowed to pray in Jerusalem if Mecca were conquered. The Prophet, peace and blessings be upon him, said to him: "Pray here." However, giving money instead of sacrificing is not sufficient, because sacrifice is an act of worship intended in itself.