Nafi' (may Allah have mercy on him) said that Abdullah bin Umar (may Allah be pleased with them both) used to say: "It is not right for a slave to spend anything from his wealth, nor to give it to anyone, without his master's permission, except that he may eat from it in a reasonable manner or clothe himself."
(The musnad hadith could, remotely, be understood to mean that the Prophet (peace and blessings be upon him) intended to encourage the owner to permit his slave to give charity on his behalf, with the reward shared between them; but the permissibility indicated by its apparent meaning is more fitting for one who wishes to follow the Sunnah. And success is from Allah.)
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7863