What obligation does the master face when unlawfully confining his contracted slave?

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The master has the obligation to choose the more beneficial option for the contracted slave regarding the period of confinement. This is either allowing the slave freedom for an equivalent period or compensating the slave with the fair rental value (Ujr Mithl) for that duration, as the cause for both remedies has occurred, granting the Mukatab the right to choose the more advantageous option.