This is Ash-Shighar Which is Forbidden

Question :

A questioner says: "A marriage took place between a man from my kin and another person. But there is some doubt regarding its validity. Because it happened that the agreement of this man took place with another person on the understanding that he would marry his daughter, and that he would give his daughter in marriage to the son of that man, and each of them made it a condition that he would pay the other what is required for the woman, such as clothes and jewelry, according to what he determined. Is a marriage such as this valid, or does it fall into the category of Ash-Shighar, which is forbidden? And if it does, what should they do now? And if it does not belong to the category of Ash-Shighar, then what is Ash-Shighar?

Answer:

A There is no doubt that this description which you have mentioned is from Ash-Shighar. Because no dowry was given in it except clothing for the woman and jewelry, and this is not a customary dowry in our time. The dowry in our time is not confined to jewelry and clothes for the woman; rather, it is accompanied by money. Accordingly, each of them has married the other for a dowry which is less than the adequate dowry. There is no doubt that this is Ash-Shighar. Because the dowry which is given to each of them has become composed of two things: the goods which are given to her, and the private parts of the other (i.e., sexual intercourse) and this is unlawful and not permissible. This is why Allah, Most High says:
"All others are lawful, provided you seek (them in marriage) with Mahr (bridal-money given by the husband to his wife at the time of marriage) from your property, desiring chastity, not committing illegal sexual intercourse"

So Allah, Most Glorified, Most High has declared the dowry to be property only (i.e., money, gold, land, etc.):

"provided you seek (them in marriage) with Mahr (dowry) from your property".

The dowry between these two men was property and private parts (i.e., sexual intercourse). For this reason, it is forbidden and it is included in the category of Ash-Shighar. However, if each of them gave the woman a dowry like the other, and each of them was suitable for the man who married her, and each of them accepted him, some of the scholars have declared it permissible. They say that it does not fall into the category of Ash-Shighar, while others hold the view that it is Ash-Shighar. So there is no doubt that preventing it is better, because in our time, the honesty and trustworthiness of people has diminished, an individual does not care about the benefit of the woman in his care, he cares only about what is beneficial to himself. Therefore, this type of marriage should be stopped completely, in order to stop the means (of sin) and prevent corruption.

Source:
Ibn 'Uthaimin
Fatawa Islamiyah, Vol. 5 Pages 415-416