Is it permissible to sell a sheep designated for the annual sacrifice to God, in order to use the money to facilitate marriage?

Slaughtering a sheep every year is not a vow. Slaughtering a sheep for Allah Almighty every year is a good deed, and persistence in this charitable act is desirable. If there is a need to sell it and benefit from its price, there is no harm in that, as long as it is not a vow.