If any person accepts Islam wholeheartedly with true intention and starts practicing Islam he will be considered a Muslim and his actions will be accepted in the eyes of Shariah even if he is forced to hide his iman (faith) due to any problem. (2) Expressing ones intention to accept Islam just in order to marry a girl is doubtful. It is necessary to get satisfaction about his iman before having the delicate and tender relation of nikah with him. (3) If he is strong enough in his iman then he can do, but practically he will face difficulties. (4) If he converts to Islam truly, starts practicing Islam and you are satisfied by his Islam and marry your daughter to him then the husband and wife will be Muslims and their children also will be Muslims. (5) This is something suspecting, seemingly he is differentiating between the faith of him and his wife. With this difference the nikah will not be valid. (6) The danger you apprehend is quite important and difficult to cast aside easily. It is essential to be satisfied about the iman of the boy, which is under suspicion here. Moreover, the girl will stay with the parents of the boy who are Sikh; this will naturally have effect on the beliefs and actions of the girl. The same effects will necessarily be transferred to her children. It is extremely serious matter to endanger ones valuable asset like iman and the iman of the children. This must be deliberated thoroughly again and again. (7) Islam is above the greed of woman and wealth; if he is embracing Islam for the sake marriage (otherwise he will not convert to Islam) then you should think twice to rely such a greedy iman. If your conscience does not allow you, you will not be sinful and you will not be considered a hindrance in his way of accepting Islam.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband