Is it permissible to give the sisters 250 pounds each instead of the agreed-upon 500 pounds, after the bankruptcy of the shop they relinquished to their brother, knowing that their father was the one who specified the lower amount?

The sisters' relinquishment of their right to the shop during their father's lifetime is not binding, and they can retract it after his death, because it is the waiving of a right before it becomes due. The father must equalize gifts and endowments among his children, unless there is an acceptable justification for preferring some of them, such as need, chronic illness, or devotion to seeking knowledge. If the sisters agree to the amount specified by the father, it is permissible to limit it to that; otherwise, they have the right to take their legal share from the inheritance or to agree upon an amount that satisfies them.