Is it permissible to help a daughter with her children's marriage and deduct the amount from her future inheritance?

It is not permissible to favor one child over another in a loan unless there is a need, or with the permission of the others. A loan is not deferrable; rather, it is due in all circumstances. If the daughter does not deserve special treatment and her brothers do not give permission, the father can help his daughter by lending to the others in the same way, or by dividing his wealth equally among them during his lifetime.