Someone left a sum of money with him as a trust and hasn’t come to collect it. Can he use it for business and be a guarantor for it?
Question

                Someone left a sum of money with me as a trust, on the basis that he would come back within a short period of time and collect it. But he has not got in touch and he has not come for a long time. I called him on the number he left with me but I did not find him there. My question is: Is it permissible for me to use this money for trade, on the basis that I will protect it and repay him when he asks for it, and will bear in full any loss that happens to this money?.
Praise be to Allah.
    Allaah and His Messenger  (peace and blessings of Allaah 
    be upon him) have commanded us to render back trusts to their owners. Allaah 
    says (interpretation of the meaning): 

    “Verily, Allaah commands that you should render back the 
    trusts to those, to whom they are due”

    [al-Nisa’ 4:58]
And the Prophet  
    (peace and blessings of Allaah be upon him) said: “Render back trusts to 
    those who entrusted you with them.” Narrated by al-Tirmidhi, 1264; classed 
    as saheeh by al-Albaani in Jaami’ al-Tirmidhi. 
So what you have to do is 
    to return the money to its owner. You have to look for him and ask for him, 
    and keep on trying to get in touch with him until you find him. 

    If you cannot find him then you have to keep his money for 
    him until he comes and asks for it. In that case you may give it in charity 
    on behalf of its owner – after doing your utmost to return it to him – then 
    if he comes along one day you should tell him what you have done. If he 
    agrees to the act of charity, he will be rewarded for it, but if he wants to 
    take his money, then you will be rewarded for the act of charity. 

    The Standing Committee for Issuing Fatwas was asked about a 
    man who left something for safekeeping with another person and did not come 
    back to collect it, and many years had passed. 

    They replied: 

    You have to do your utmost to find the owner of this trust. 
    If you find him or one of his heirs then give it to him. If you cannot find 
    him then donate it to some charitable cause, with the intention of giving 
    charity on behalf of the owner. If the owner or his heir comes along after 
    that, then tell him what you have done. If he approves of that, all well and 
    good, but if he does not approve, give him the equivalent amount, and you 
    will have the reward for what you gave in charity, in sha Allaah. 

    Fataawa al-Lajnah al-Daa’imah, 
    15/404 
The Standing Committee 
    was also asked about a man who left a sum of money with another man for 
    safekeeping, then he went back to his own country and did not come back. The 
    person with whom the money was left does not know what town the man came 
    from. What should he do with this money? 

    They replied: 
If the situation is as 
    you describe, then if you wish you may keep it and try hard to find out 
    where this man is, or if you wish you may give it in charity to the poor, or 
    donate it to some other charitable cause with the intention that the reward 
    for that should go to its owner. If the owner or his heir comes to you after 
    that, then tell him what happened. If he approves, all well and good, 
    otherwise give him the same amount of money, and you will have the reward in 
    sha Allaah. 

    Fataawa al-Lajnah al-Daa’imah, 
    15/406.