Paying the girl’s dowry with her brother’s money
Question

                Suppose if a parents have Son and Daughter and after long search they found a husband for their daughters but bridegroom is requesting for a dowry but the parents are not affordable to pay them, so they try to get dowry for their son in order to pay for their daughter. Certainly they are not going to use this money except to give dowry for their daughter marriage.  

I like to through some lights and give us proper guidence to handle this situation.
Praise be to Allah.It is very strange indeed that in some 
    countries the dowry is paid by the bride or her family to the groom or his family. This is contrary to the Islamic principle, because the 
    Messenger  (peace and blessings of Allaah be upon him) commanded a man who was getting married to give a dowry even if it was an iron ring, 
    and when he did not even have that, he made the dowry that he would teach her what he had memorized of the Qur’aan. 
What is important is that in the marriage 
    contract something should be mentioned as the dowry, even if it is little. 
It was narrated that Sahl ibn Sa’d said: A 
    woman came to the Messenger of Allaah  (peace and blessings of Allaah be upon him) and said: “I give myself to you (in marriage).” He paused 
    for a while, then a man said, “Marry her to me, if you have no need of her.” He said, “Do you have anything you can give to her as a dowry?” He 
    said, “I do not have anything but my izaar (lower garment).”  He said, “If you give that to her, you will not have any lower garment. Think of 
    something.” He said, “I cannot think of anything.” He said, “Think of something, even if it is an iron ring.” But he did not have anything. (The 
    Prophet  (peace and blessings of Allaah be upon him)) said, “Have you memorized anything of the Qur’aan?” He said, “Yes, Soorah such and such, 
    and Soorah such and such” – and he named the soorahs. He said: “We marry her to you on the basis of what you have memorized of the Qur’aan [i.e., 
    that you teach it to her].” 
(Narrated by al-Bukhaari, 4842; Muslim, 
    1425) 
This hadeeth indicates that the Messenger 
     (peace and blessings of Allaah be upon him) did not agree to the man marrying without a dowry to give to the woman; he did not ask the woman 
    for anything. 
Moreover, the concept of qawaamah (being 
    maintainers and protectors of women) that Allaah has enjoined on men implies that the man is the one who gives the mahr to the woman, because he 
    is her supporter and she is his dependent.  
Allaah says (interpretation of the meaning): 
“Men are the protectors and maintainers of 
    women, because Allaah has made one of them to excel the other, and because they spend (to support them) from their means”
[al-Nisa’ 4:34]
Moreover, the mahr is the woman’s right, because the man enjoys 
    intimate relations with her, and the mahr is paid in return for that intimacy. 
Allaah says (interpretation of the meaning): 
“…so with those of whom you have enjoyed 
    sexual relations, give them their Mahr as prescribed…”
[al-Nisa’ 4:24]
Imam Ibn Katheer (may Allaah have mercy on him) said: 
The phrase “so with those of whom you have 
    enjoyed sexual relations, give them their Mahr as prescribed” means, as you are going to enjoy intimacy with them, then give them their 
    dowries in return for that, as Allaah says (interpretation of the meaning): 
“And how could you take it (back) while 
    you have gone in unto each other”
[al-Nisa’ 4:21] 
“And give to the women (whom you marry) 
    their Mahr (obligatory bridal-money given by the husband to his wife at the time of marriage) with a good heart”
[al-Nisa’ 4:4] 
“And it is not lawful for you (men) to 
    take back (from your wives) any of your Mahr (bridal-money given by the husband to his wife at the time of marriage) which you have given them”
[al-Baqarah 2:229]
Tafseer Ibn Katheer, 
    1/475 
It was narrated from ‘Aa’ishah that the Messenger of Allaah  
    (peace and blessings of Allaah be upon him) said: “Any woman who gets married without the permission of her guardian, her marriage is invalid, her 
    marriage is invalid, her marriage is invalid. If the marriage has been consummated then the mahr is hers, because she has permitted (the man) to 
    be intimate with her. If (the guardian) refuses to arrange her marriage, then the ruler is the guardian of the one who has no guardian.” 
Abu ‘Eesa al-Tirmidhi said: this is a hasan 
    hadeeth. 
From this we can see that the dowry is to be 
    paid by the man to the woman, not by the woman to the man. 
Shaykh ‘Abd-Allaah ibn Qa’ood said: “The mahr 
    is the right of the wife, and it must be defined. The wife or her family do not have to pay anything unless they do so voluntarily.” 
Based on this, it is not permissible for you 
    to take from your son’s money and give it as a dowry for your daughter. Shaykh al-Barraak said: 
“If it is not permissible for the son to take 
    the dowry in the first place, it is not permissible to take it for the daughter.”  
If you fear Allaah, Allaah will grant a way 
    out for your daughter. So she has to be patient and seek reward, and turn to Allaah and make du’aa’. If a person thinks of Allaah in positive 
    terms, he will find that Allaah is as he thinks. 
The scholars, prominent figures and the 
    ordinary people in your country have to strive to change this bad custom and follow the Sunnah, and to do the right thing and follow the right way 
    which no one is permitted to go against. That is by establishing proof to the people from the Qur’aan and Sunnah and the words of the scholars. 
And Allaah knows best.