What conditions must be met in the case of mandatory set-off? 
In the case of mandatory set-off, each of the parties must be both the obligor and the obligee of the other, and the obligations must be of the same type and description, must be equally due and of equal strength or weakness, and the making of the set-off must not be prejudicial to the rights of third parties, whether the cause of the arising of the obligations is the same or different.