Is it considered usury for a master to lend his slave money on the condition that he repays it with an increase, knowing that the master owns the slave and has the right to dispose of him?

The scholars answered your question that a Muslim should only ask about matters that are beneficial for this world and the hereafter. Allah has forbidden usury (riba) in His explicit book and through the tongue of His Messenger, peace and blessings be upon him. Some scholars have gone to the extent of saying that there is no difference in the prohibition between usury occurring between a slave and his master, or between two strangers. Scholars differed on this issue; Ibn Abbas and those who agreed with him among the scholars, including Imam Malik, held the view that there is no usury between a slave and his master, while Jabir and those who agreed with him held the view of the absolute prohibition of usury, including what occurs between a slave and his master.