What is the solution for someone who borrowed from banks for his daughters' marriages and now discovers that the loan involves an element of usury (riba) and fears that he will be afflicted with harm while being guilty of this sin?

Borrowing from banks with a stipulated increase is explicit usury (riba) and is not permissible unless out of necessity, and preparing daughters for marriage is not considered a necessity.

One must hasten to repent by abandoning the sin, regretting it, and resolving not to return to it. Part of repentance is returning the principal amount of the loan without the usurious increase, if possible. If that is not feasible, then your repentance and seeking forgiveness are sufficient.

A penitent person will not be punished for their sin in this world or in the hereafter.