Praise be to Allah.
If a person finds a lost sheep, then he should take it and do whatever is best, whether that is eating it and estimating its value, or selling it and keeping the money, or keeping it for the prescribed period of time whilst trying to find its owner. Leaving it and not taking it exposes it to destruction. Then if its owner comes along, the person can either return it or pay him its value or the price he got for selling it. If the owner does not show up, then it belongs to the one who found it.