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.
A person who wants to marry is obliged to give dowry to the bride. This wealth should be something valuable which is permitted to benefit from; this is called a dowry whether it is paid promptly or delayed to an appointed time.
There is no harm for the bride to give some money to her fiancé to give it as a dowry to her in order to avoid any embarrassments [in front of the people] but he should give it back to her in the future. There is no harm for this matter to be kept as a secret between the fiancée and her fiancée, but this right should be documented. For instance the amount of money he owes to her should be written on a piece of paper in order to protect her right; if such a writing can be witnessed by two witnesses then this is better. There is also no harm also for her to ask for her right after marriage and he is obliged to repay you.
Allaah Knows best.