He is investing someone else’s wealth and will bear half of the losses
Question

                A man is investing someone else’s wealth. If he makes money, is it permissible for him to give the investors 80% of the profit and keep the rest as a fee, but in the event of a loss, 50% will be borne by each party? If it is not permissible, what is the shar’i way of doing it?.
Praise be to Allah.Firstly: 
The one who invests the 
    wealth of others either takes part by means of his work only, so that the 
    money belongs entirely to someone else, or he takes part by means of his 
    work and his wealth. The former is called a mudaarabah transaction, in which 
    the profits are shared according to whatever he has agreed upon with the 
    owners of the money. It may be 20% or more or less, but the rate must be 
    stipulated in the mudaarabah contract. 
As for losses, he does not 
    bear any of them; the losses are borne entirely by the owner of the capital, 
    and the worker loses his work and effort, unless the loss is due to some 
    shortcoming or transgression on the part of the worker, in which case he has 
    to bear the loss. 
If it is stipulated in the 
    mudaarabah contract that any loss or part of it – such as 50% - is to be 
    borne by the worker, then this condition is invalid and there is a 
    difference of scholarly opinion as to whether the contract itself is valid 
    or invalid. 
Secondly: 
If you are contributing 
    both your wealth and your work, then the profits are to be shared according 
    to the agreement, as stated above. 
As for losses, they are to 
    be borne according to the capital invested. If you put in one-tenth of the 
    capital, for example, then you must bear one-tenth of the loss; if you put 
    in one half of the capital, then you must bear one half of the loss, and so 
    on. 
Among the comments of the 
    scholars about the worker in a mudaarabah contract not having to bear any 
    financial loss, and the partners bearing losses according to their shares 
    are the following: 
Ibn Qudaamah (may Allaah 
    have mercy on him) said in al-Mughni (5/22): The loss is to be borne 
    by each partner according to his share of the wealth; if the shares (of two 
    partners) were equal then they each bear half of the loss. If there were 
    three shares then they each bear one third of the loss. We do not know of 
    any difference of scholarly opinion concerning this matter, and it is the 
    view of Abu Haneefah, al-Shaafa’i and others. 
The loss in a mudaarabah 
    contract affects the (invested) money only, and the worker does not bear any 
    loss, because the loss affects the capital, which has to do with the owner 
    of that capital, and the worker has nothing to do with it, so the loss 
    affects the investor’s wealth only; but they share any growth that takes 
    place. End quote. 
He also said: If it is 
    stipulated that the worker is liable for the money or for part of any loss, 
    this condition is invalid, and we do not know of any dissenting opinion, but 
    the contract is still valid. This was stated by Ahmad and it was the view of 
    Abu Haneefah and Maalik. 
It was narrated from Ahmad 
    that the contract is invalidated thereby, and this was narrated from 
    al-Shaafa’i, because it is an invalid condition which invalidates the 
    mudaarabah contract. But the former view is more correct.  
End quote from al-Mughni 
    (5/40). 
And Allaah knows best.