Is the amount you received through the “upon request” lawsuit considered Islamically permissible (halal), or must it be returned to your husband after the completion of the amicable divorce proceedings, especially since you will be waiving your deferred mahr?

First: The increase in the dowry is annexed to the original dowry, whether it was before or after the contract. Thus, your dowry becomes four million: two million immediate and two million deferred.

Second: There is no harm in you demanding the two million deferred dowry, and it is permissible for you to seek divorce due to your husband's transgression. You are not required to return anything you have taken.