Once a person receives a gift, he becomes the owner and therefore has the right to use it according to the rulings of Shari`ah. Therefore, the sheep are the property of the one to whom they were given as charity and he can give them in charity or slaughter Ud-hiyah (sacrificial animal offered by non-pilgrims) as he likes. Also, the first person who gave the sheep as a gift to the second person will be rewarded for his gift if he intended reward from Allah (Exalted be He). May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.