Musannaf Ibn Abi Shayba 13:1204

Abu Bakr narrated to us, he said: Jarir narrated to us, from al-Mughirah, from al-Harith, regarding a man who had dirhams owed to him by another man, and he said to him: "Buy something for me with them." He said: "There is no harm in that." If the thing he bought perishes, then his proof is that he did indeed buy it; otherwise, he is not believed that he bought it for him. If it is a *mudarabah* (profit-sharing) contract, then he should not buy anything for him with it until he takes possession of it or gives it to a guardian of his.