He borrows money from his friend and returns the money to him with a share of the profit
Question

                I work as a trader. Is it permissible for me to borrow from a Christian friend of mine, and agree with him to return the money to him with a proportion of the profit? I specify the time period and the profit myself.
Praise be to Allah.It is not permissible to 
    take a Christian as a close friend, because Allaah has stated that there can 
    be no close ties between them, as He says (interpretation of the meaning): 
“You (O Muhammad
    صلى الله عليه وسلم) will not find any 
    people who believe in Allaah and the Last Day, making friendship with those 
    who oppose Allaah and His Messenger (Muhammad 
    صلى الله عليه وسلم), even though they were their fathers or their 
    sons or their brothers or their kindred (people)”
[al-Mujaadilah 58:22] 
“O you who believe! Take 
    not for Awliyaa’ (protectors or helpers or friends) disbelievers instead of 
    believers. Do you wish to offer Allaah a manifest proof against yourselves?”
[al-Nisa’ 4:144] 
“O you who believe! Take 
    not the Jews and the Christians as Awliyaa’ (friends, protectors, helpers), 
    they are but Awliyaa’ of each other. And if any amongst you takes them (as 
    Awliyaa’), then surely, he is one of them. Verily, Allaah guides not those 
    people who are the Zaalimoon (polytheists and wrongdoers and unjust)”
[al-Maa’idah 5:51] 
“O you who believe! Take 
    not as Awliyaa’ (supporters and helpers) your fathers and your brothers if 
    they prefer disbelief to Belief. And whoever of you does so, then he is one 
    of the Zaalimoon (wrongdoers)”
[al-Tawbah 9:23]
“O you who believe! Take 
    not as (your) Bitaanah (advisors, consultants, protectors, helpers, friends) 
    those outside your religion (pagans, Jews, Christians, and hypocrites) since 
    they will not fail to do their best to corrupt you. They desire to harm you 
    severely. Hatred has already appeared from their mouths, but what their 
    breasts conceal is far worse. Indeed We have made plain to you the Ayaat 
    (proofs, evidences, verses) if you understand”
[Aal ‘Imraan 3:118]
And the Prophet  (peace 
    and blessings of Allaah be upon him) said: “Do not keep company with anyone 
    but a believer.” narrated by Abu Dawood (4832) and classed as hasan by 
    al-Albaani in Saheeh Abi Dawood. 
And there are other texts 
    which indicate that it is haraam to take kaafirs as close friends or love 
    them and choose them as confidantes and close friends. See also the answer 
    to question no. 2179. 
Secondly: 
It is permissible to 
    interact with the kuffaar by buying, selling, lending, pledging and so on, 
    so there is nothing wrong with you borrowing money from him, but it is not 
    permissible for that to involve interest, because that is riba which is 
    haraam. If a person borrows one hundred from someone on the basis that he 
    will give him back one hundred and ten, for example, then he has fallen into 
    riba which is one of the worst of major sins, because every loan which 
    brings a benefit is riba. 
The permissible way to do 
    it in this case is to take the money from him and give it back to him 
    without paying anything extra. This is the good kind of loan which is 
    acceptable in sharee’ah. 
It is permissible for him 
    to join you as a (business) partner, so he can give you the money for you to 
    use in business, on the basis that he will have a set share of the profit 
    which Allaah decrees for you, such as five or ten percent of your profits. 
    This share must be agreed upon before the partnership begins, and it is not 
    permissible for it to remain unknown. You can agree on what the share should 
    be, whether it is a little or a lot. 
So you can agree that you 
    will invest his money for a year, for example, and he will have a quarter of 
    the profit or 10%. Then whatever profit you make, whether it is a little or 
    a lot, give him his share. If you do not make any profit, then he is not 
    entitled to anything. This is what is called mudaarabah (profit sharing) in 
    sharee’ah. 
In the event of loss, the 
    loss is borne by all partners according to each partner’s share of the 
    capital, and it is not permissible to agree to share the loss in any other 
    manner. 
And Allaah knows best.