What is the justification for the master retaining the wealth of a returned mukātab?

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The justification for the master retaining the wealth is based on the action of Ibn Umar, who, when returning a mukātab to slavery, kept what he had received from him. Furthermore, the wealth was taken for the slave’s personal need (to secure his freedom), and thus, what was received should not be returned, analogous to the poor and needy (Miskeen). This differs from a Ghazi or Gharim who takes wealth based on a need for public benefit, in which case, if the need is satisfied, the remainder might be returned.