Must I pay the deferred mahr now, knowing that the prevailing custom is to pay it upon divorce as a guarantee for the wife, and would I be considered sinful if I do not pay it?

The deferred Mahr (dowry) is a debt and a right due to the woman. If its term becomes due, it must be paid if the husband is able. The husband is sinful for delaying it, because procrastination is injustice. However, if it was not deferred, and custom dictates that she is not entitled to it except upon death or divorce, then she does not have the right to demand it until one of those events occurs. It is preferable to pay it if the husband is able, because it is an established right upon him.