Is it permissible to give Zakat al-Mal (obligatory charity on wealth) to the daughter's fiancé, who is unable to complete the marital home, with the fear that the marriage may not be finalized and the sin that would result from that?

If your daughter's suitor is poor and in need, there is no harm in giving him from zakat funds to build a house that is not extravagant. The money becomes his property to do with as he wishes, whether the marriage takes place or not. However, it is not permissible to give him zakat on condition that the house, or a part of it, belongs to your daughter, because a person is not allowed to give zakat from their wealth to their ascendants or descendants.