Is it permissible for a mukatab (indentured servant purchasing freedom) to hasten the payment of his servitude and forgo a portion of the remaining amount owed to his master?

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It is permissible for the mukatab to pay his master early while foregoing a portion of his remaining servitude payment. This is an exception because the transaction involves the servant selling a portion of his wealth (his future labor/value) for another portion, allowing for leniency. Furthermore, since this transaction serves as a means toward manumission (freedom), leniency is permitted, unlike other similar transactions.