Is it permissible for a husband to pay off his wife's debt from Zakat money if she incurred it before marriage?

Yes, there is no harm if a husband gives his zakat to his wife to pay off her debt, because he is not obliged to pay her debt. However, it is impermissible to give zakat to the wife if the purpose is to waive a mandatory expenditure that he owes her. The jurists have agreed on the permissibility of this if the giving is for the purpose of settling the wife's debt or spending on others.