What is the ruling when a Mukatab (indentured writer) claims to have paid the installments for their manumission, but the master denies it?

Primary text

When a Mukatab claims fulfillment of the payment installments required for his freedom ('Aṭaqt' - I am freed), and the master denies it, the statement of the master is accepted, provided he swears an oath. This is because the master is the denier of the claim made by the Mukatab, and the ruling favors the statement of the denier.