Is he sinning if he keeps one of his wives with him and sends the other back to his homeland?
Question

                One of my brothers has two wives. He lives abroad. Due to his difficult financial circumstances he kept his first wife with him and told the second wife to stay with her family for some time until this difficulty passes, as he cannot maintain two homes for the two wives, especially abroad. He always asks and fears that he is unfair with the second wife. Although he gave her money before she traveled and she accepted what he asked for. He feels guilty. Is he sinful? Is he considered unfair or not?.
Praise be to Allah.
    The 
    husband must treat co-wives fairly with regard to staying overnight with 
    them, spending and providing accommodation. This fairness is a condition of 
    plural marriage being permissible. Allaah says (interpretation of the 
    meaning): 

    “marry (other) women of your choice, two or three, or four; but if you fear 
    that you shall not be able to deal justly (with them), then only one”

    [al-Nisa’ 4:3]

    Abu 
    Dawood (2133) narrated that the Prophet  (peace and blessings of Allaah 
    be upon him) said: “Whoever has two wives and inclines more towards one of 
    them will come on the Day of Resurrection with half of his body leaning.” 
    Classed as saheeh by al-Albaani in Irwa’ al-Ghaleel (2017). 

    The 
    husband has no right to show preference to one of his wives by keeping her 
    with him and sending the other one back to her family. Rather he must treat 
    them fairly and keep one of them for a while and send the other one away, 
    then reverse the situation, and so on, so that each of them will have the 
    rights that Allaah has prescribed for her. 

    If 
    one of the two wives agrees to give up her rights and does not mind if the 
    other wife is favoured over her, there is nothing wrong with that, but that 
    is subject to the condition that it be with her consent, without any force 
    or compulsion from the husband, and whenever the man is able to treat both 
    wives fairly, that is what he must do. 

    We 
    put your question to Shaykh Dr. Khaalid al-Mushayqih (may Allaah preserve 
    him) and he said: Yes, he must treat his wives fairly, because Allaah says 
    (interpretation of the meaning): “and live with them honourably” 
    [al-Nisa’ 4:19]. Living with them honourably does not include keeping 
    one and sending the other away, rather he must treat them fairly, either by 
    keeping one for a while then sending her away and bringing the other, and so 
    on, or by coming to some agreement with the one whom he sent away that he 
    will come and visit her in her homeland or will give her some money or work 
    out some other agreement and so on. End quote. 

    And Allaah 
    knows best.