The obligation to treat co-wives fairly in giving gifts
Question

                I have two wives, and I want to buy some gold for one of them as a gift. Is it permissible for me to do that, or would this be unfair treatment of co-wives? Please note that I do not fall short in giving the other wife her rights.
Praise be to Allah.
    Whoever has two or more wives has to treat them fairly. It is 
    not permissible for him to single out one of his wives with regard to 
    spending, accommodation or spending the night, to the exclusion of the 
    others. There is a stern warning issued to the one who has two wives and 
    does not treat them fairly. It was narrated that Abu Hurayrah (may Allaah be 
    pleased with him) said: The Messenger of Allaah  (peace and blessings of 
    Allaah be upon him) said: “Whoever has two wives and inclines more towards 
    one of them than the other, will come on the Day of Resurrection with half 
    of his body falling.”  According to another version: “Dragging half of his 
    body which will be falling or leaning.” Narrated by Imam Ahmad in 
    al-Musnad, 2/295, 347, 471. A similar hadeeth was also narrated by 
    al-Nasaa’i and Ibn Maajah in their Sunans. 

    And it was narrated from Abu Hurayrah (may Allaah be pleased 
    with him) 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.” Narrated by 
    Abu Dawood in his Sunan, 2/601. Al-Tirmidhi narrated something 
    similar in al-Jaami’. 

    This evidence indicates that it is obligatory to treat 
    co-wives fairly and equally, and that it is haraam for the husband to 
    incline more to one of them in a way that will hurt the other. This does not 
    include the inclination of the heart, for a person has no control over the 
    inclination of the heart. Hence the Messenger of Allaah  (peace and 
    blessings of Allaah be upon him) used to divide his time equally among his 
    wives, and he said: “O Allaah, this is my division of that which I can 
    control, so do not take me to task for that over which You have control and 
    I have no control” [meaning love in the heart]. 
Based on this, it is not 
    permissible for this husband to single out one of his wives with regard to 
    something over which he has control. If he gives a house etc to one of his 
    wives, he must treat all his wives equally in that regard, and give a 
    similar gift or something of equal value to all of them, unless the second 
    wife allows that.