John Calvin Commentary Philemon 1:18

"But if he hath wronged the at all, or oweth [thee] aught, put that to mine account;" — Philemon 1:18 (ASV)

If in anything he hath done thee injury. Hence we may infer that Onesimus had likewise stolen something from his master, as was customary with fugitives; and yet he softens the criminality of the act, by adding, or if he oweth thee anything. Not only was there a bond between them recognized by civil law, but the slave had become indebted to his master by the wrong which he had inflicted on him. So much the greater, therefore, was the kindness of Paul, who was even ready to give satisfaction for a crime.