Is not paying a dowry to the man who proposes to my sister forbidden?

The mahr (dowry) is a right of the wife over her husband, and it is not permissible for the husband to stipulate that he should take a mahr from her or from her guardian, for this is consuming wealth unjustly. If the husband stipulates this and the nikah (marriage contract) is concluded, the nikah is valid according to the majority of scholars, and the husband must present a mahr, even if it is small.

Ali (may Allah be pleased with him) issued a fatwa that a wife should not give sadaqah (charity) to men, and that this is contrary to the Sunnah. The Islamic Fiqh Academy decided that the custom of "dowry" (an amount paid by the bride for marriage) is evil and reprehensible, and contrary to the Quran, Sunnah, and the consensus of scholars. They affirmed that marriage with the existence of this custom is legally valid according to the majority of scholars, and that the children are legitimate.

The Academy also emphasized that the mahr is a right of the wife and the husband must pay it, whether it is prompt or deferred. They prohibited marriage without a sadaq (bridal gift) and recommended reducing the mahr and facilitating marriage.

The Islamic Fiqh Academy in India concluded that the dowry is absolutely forbidden, and that Islam has not placed any financial burden on the wife or her guardians. Rather, the mahr and maintenance are the responsibility of the husband. What is presented to the wife at the time of the marriage contract by her family or by the husband and his family is her property and it is not permissible to reclaim it or dispose of it without her consent.

In summary, the husband's request for the wife to pay him the mahr is forbidden in Islam. However, if marrying the sister is not possible except through that, then it is coercion, and there is no sin upon the guardian.