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.
This person should fulfill his vow according to the manner that he committed himself to. If he imposed upon himself to slaughter a sheep, then he has to do so. It does not free him of his liability to buy a ready-slaughtered sheep because the reality of this action is buying meat, not slaughtering a sheep.
This person is required to fulfill his vow whenever this is possible for him as long as he did not appoint a specific timing for doing so. If he is permanently unable to fulfill it, then he should offer an expiation for oath breaking.
Allaah Knows best.