All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
When one makes a vow to do something lawful according to the Sharee'ah, then he is required to fulfill it according to the way that he specified it. It is not permissible to give something else unless the person is unable to obtain the thing he vowed to give. Consequently, you are not permitted to give the value of the sheep instead of its meat. You are required to slaughter the sheep, unless you are unable to get one. In the latter case, giving the value clears you of your liability. Allaah Almighty says (what means):
• {Allaah does not charge a soul except [with that within] its capacity.} [Quran 2: 286]
• {Allaah does not charge a soul except [according to] what He has given it.} [Quran 65: 7]
Allaah Knows best.