If his partner is absent from work, is it permissible for him to deduct from his earnings for that reason?
Question

                I have a partner in the office where I work; we agreed to split the earnings evenly every month, but he was absent a great deal during the month, more than four times, apart from Fridays which is our day off. Please note that I am responsible for running the office, in addition to my work with him, and he does not have any kind of responsibility apart from his own work. If he is absent from work for one day, am I within my rights to deduct that day from the monthly earnings?
Praise be to Allah.
      Firstly: 

    The basic 
    principle in the case of a dispute between two partners or parties to an 
    agreement is that they should refer to the conditions agreed upon in the 
    contract between them, so long as these conditions are not contrary to 
    Islamic teaching, because Allah, may He be exalted, says (interpretation of 
    the meaning): “O 
    you who believe! Fulfill (your) obligations” 
    [al-Maa’idah 5:1]. 
    And the Prophet (blessings and peace of Allah be upon him) said: “The 
    Muslims are bound by their conditions, except a condition that forbids 
    something permissible or permits something forbidden.”

    Narrated by 
    at-Tirmidhi (1352) and Abu Dawood (3594); classed as saheeh by al-Albaani in
    Saheeh at-Tirmidhi. 

    Based on that, if 
    there was an agreement between you to deduct from the earnings in the event 
    of one of the two partners being absent, then there is no doubt that this 
    deduction is permissible. 

    Similarly, if the 
    normal practice in companies like yours is that if one of the partners is 
    absent, a deduction is made in that case, and this is well known to you and 
    your partner, then there is nothing wrong with this deduction, because the 
    basic fiqhi principle is that that which is well known according to custom 
    is like a condition that has been stipulated. 

    See: Ghamz 
    ‘Uyoon al-Basaa’ir fi Sharh al-Ashbaah wa’n-Nazaa’ir, 4/206 

    Secondly: 

    If there was no 
    agreement in writing concerning such situations, and there is no such 
    consistent practice in the culture of similar companies, then the case of 
    the partner who was absent must be one of two possibilities: 

    1.

    That he was absent 
    for a valid reason, such as sickness or some other problem. In that case no 
    deduction should be made from his earnings, but you are within your rights 
    to require him to appoint a deputy, at his own expense, to do his work, 
    without expecting the company to bear the cost of paying that deputy. If he 
    refuses to do so, then you are within your rights to cancel the partnership, 
    because the agreement was to work together, and if he does not adhere to 
    that, then you have the right to cancel. 

    2.

    That his absence 
    was not for a valid reason; rather it was due to carelessness or a reason 
    that he could have overcome. In this case there was a difference of opinion 
    among the scholars – may Allah have mercy on them – as to whether the 
    partner is entitled to his earnings in full, or not. 

    Some of the 
    scholars are of the view that the partner is entitled to his full share of 
    the earnings, according to what is agreed upon in the contract, as it is 
    sufficient that the other partner has the right to cancel the contract if 
    any of the conditions are not met. If he did not decide to cancel it before 
    the earnings were known, then the earnings are to be shared between them in 
    accordance with what they agreed upon. 

    It says in 
    Majallat al-Ahkaam al-‘Adliyyah: Item no. 1349: 

    Entitlement to a 
    share of the earnings is based on the conditions stipulated in the 
    partnership contract, not on the work that was done. Based on that, if the 
    partner whose stipulated role was to work did not work as stipulated, he is 
    still to be regarded as if he did do the work. For example, if it was 
    stipulated that both partners in a valid partnership should work, and only 
    one of them did the work and the other did not, with or without a valid 
    excuse, then because they are deputies of one another, if one partner does 
    the work it is as if the other partner did it, therefore the earnings are to 
    be shared between them in to the manner stipulated. 

    Ibn Qudaamah (may 
    Allah have mercy on him) said: 

    If one of them 
    works but the other does not, the earnings are to be shared between them, 
    whether (the latter) did not work because of sickness or otherwise. If one 
    of them asked the other to work with him, or to appoint someone else in his 
    stead, he is within his rights to do so. If (the latter) refuses, then (the 
    former) is within his rights to cancel the agreement.

    End quote from 
    al-Mughni, 5/7 

    The second view 
    concerning this issue is that the partner (who does not do his work) does 
    not deserve his full share of the earnings because he has not upheld his 
    part of the agreement. 

    Al-Mirdaawi (may 
    Allah have mercy on him) said: 

    What is understood 
    from the words “if one of them is sick, the earnings are for both of them” 
    is that if he did not do the work for no good reason then the earnings are 
    not to be shared between them. This is one of the two views [of the 
    Hanbalis], and the author [i.e., Ibn Qudaamah] thinks that this is the 
    correct view. 

    End quote from 
    al-Insaaf, 5/461 

    Shaykh Ibn 
    ‘Uthaymeen (may Allah have mercy on him) said: 

    The more correct 
    view concerning this issue is that if he did not do the work for no valid 
    reason, then he does not deserve the earnings of the time during which he 
    did not work, with no valid excuse. End quote from ash-Sharh al-Mumti‘, 
    9/436 

    Whatever the case, 
    whether his absence was for a valid reason or not, you are within your 
    rights to cancel the previous agreement and make a new agreement in which 
    you state how earnings are to be shared out if either of the two partners is 
    absent. This is more prudent for both of you, and less likely to lead to 
    conflicts or create doubts concerning the honesty of either partner. 

    And Allah knows 
    best.