All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
First of all, it is not permissible to marry a Hindu girl before she embraces Islam. According to the agreement of all the scholars if a man marries her before she embraces Islam, the marriage contract is void even if he sets a condition for her to become a Muslim after the marriage contract. Allaah Says (what means): {And do not marry Al-Mushrikaat (idolatresses, etc.) till they believe (worship Allaah Alone).}[Quran 2:221]. For the marriage contract with her to be valid, and to be permissible to marry her, it is a condition that she really embraces Islam as it is not sufficient for her just to intend to embrace Islam. For more details please refer to Fatwa 88531.
Allaah Knows best.