What are the conditions for the permissibility of adding an extra charge above the agreed Mukharajah on a capable slave?

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If a slave capable of earning makes a compromise (Mukharajah) and the master subsequently places an additional charge upon him equivalent to what remains after his expenses and the initial Mukharajah payment, this is permissible because both parties derive benefit from it, as the slave is motivated to earn more, potentially increasing his own provisions. However, if the additional charge placed upon him exceeds what he earns beyond his expenses and initial Mukharajah, it is not permissible.