Should one fulfill his vow of donating an animal to the poor and needy with one sheep although he had repeated the vow several times?
Each vow has to be fulfilled, and repeating the words of the same vow for confirmation renders its fulfillment obligatory as one vow. As for multiple vows, they have to be fulfilled as well.
Is permissible to buy/sell with non-Muslim?
Yes buying/selling for a Muslim with non-Muslim is permissible as regards what is lawful.
Should missed Sunnah prayer be made up?
Making up missed voluntary acts of worship is from Sunnah.
I`m a Muslim woman and have fallen in love with a Christian man. Supposing that he embraced Islam. Is it permissible that he proposes to me?
Praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
The relationship between you two is forbidden; however, if he wants to embrace Islam then he should pay a visit to a Sharia court. Once he becomes a Muslim, we will address the matter of proposing to you. And Allah The Almighty Knows Best.