Partner took money from the company without his partner’s knowledge
Question

                I was a partner with a friend of mine in a store and I noticed that he was cheating me, so in return I started cheating him. In other words, each of us was taking something without the knowledge of the other. How can I restore his rights to him, noting that I do not know the exact amount? Is it sufficient for me to ask him for forgiveness in general, i.e., please forgive me for that which Allah knows, and I will forgive you likewise? Is this sufficient to discharge what is owed?
Praise be to Allah.
    Firstly: 

    Cheating or treachery is not a characteristic of the 
    believers; rather it is a characteristic of the hypocrites. The Prophet 
    (blessings and peace of Allah be upon him) said: “There are four things, 
    whoever has all of them is a pure hypocrite and whoever has one of them has 
    one of the characteristics of hypocrisy, until he gives it up: When he 
    speaks he lies, when he is entrusted with something he betrays that trust…”

    Narrated by al-Bukhaari, 33; Muslim, 88 

    According to the hadeeth of Abu Hurayrah (may Allah be 
    pleased with him), he said: “The adulterer is not a believer at the time 
    when he is committing adultery; the thief is not a believer at the time when 
    he is stealing; the one who misappropriates something from the war booty is 
    not a believer at the time when he is misappropriating it, so beware, 
    beware.” Narrated by Muslim, 86. 

    An-Nawawi (may Allah have mercy on him) said: 
    “Misappropriating something from the war booty” is a kind of betrayal. End 
    quote from Sharh Muslim, 2/45. 

    Betrayal is a blameworthy characteristic; the one who does it 
    is not beloved to Allah, may He be exalted: “Verily, 
    Allah does not like anyone who is a betrayer of his trust, and indulges in 
    crime” [an-Nisa’ 4:107]. 
    And there are other verses which indicate that this characteristic is 
    hateful to Allah, may He be exalted, and contrary to sound human nature. 

    The author of az-Zawaajir regarded it as a major sin. He (may 
    Allah have mercy on him) said: 

    The one hundred and eighty-seventh major sin is devouring 
    wealth by means of corrupt transactions and all kinds of haraam earnings. 
    Allah, may He be exalted, says (interpretation of the meaning): 
    “O you who believe! Eat not up your property among yourselves 
    unjustly” [an-Nisa’ 4:29]. Ibn ‘Abbaas said: This 
    refers to what a person takes without giving something in return. That is 
    because consuming another person’s wealth unjustly includes everything that 
    is taken unjustly, whether it is by unlawful means such as plunder, 
    treachery or theft, or by means of a plot and cheating, such as that which 
    is taken by means of false contracts.

    End quote from az-Zawaajir bi Iqtifraaf al-Kabaa’ir, 
    2/106 

    It was narrated that Ibn Mas‘ood (may Allah be pleased with 
    him) said: The Messenger of Allah (blessings and peace of Allah be upon him) 
    said: “Whoever deceives us is not one of us, and plots and deceit lead to 
    the Fire.” Narrated by at-Tabaraani in al-Kabeer and as-Sagheer 
    with a jayyid isnaad; and by Ibn Hibbaan in his Saheeh. See: 
    Saheeh at-Targheeb wa’t-Tarheeb by Shaykh al-Albaani (may Allah have 
    mercy on him), 2/159 

    Secondly: 

    You said: “I noticed that he was cheating me…” 

    Our response is that the basic principle concerning the 
    Muslim is that he is honest, unless there is proof that he is otherwise. If 
    there is no such proof, then the basic principle remains that he is honest 
    and innocent of treachery. 

    Based on that, what you must do first of all is repent 
    sincerely to Allah, may He be exalted; see the answer to question no.
    46683. 

    Thirdly: 

    You must return the money that was taken unlawfully, and you 
    will not have done all that is required of you except by returning it, 
    unless you tell him what has happened, and he pardons you, in which case it 
    does not matter. If he accepts what you say about having taken some of the 
    money, without asking how much you took, and forgives you, that is good 
    enough.

    some of the money, without asking him to tell you how much it 
    was and?? 

    You should also pardon him in the same manner, without asking 
    him how much he took. 

    You can also return the money without telling him; if you do 
    not know how much it was then you have to do your best to work it out and 
    give what you think most likely is the amount you took. For more 
    information, please see the answer to question no.
    40019 

    And Allah knows best.