Of mine own will (hekōn) - not of mine own will (akōn). Both common adjectives, but only here in N.T. save hekōn, also in Rom 8:20. The argument is not wholly clear. Paul’s call was so clear that he certainly did his work willingly and so had a reward (see Mat 6:1 for misthos); but the only reward that he had for his willing work (Marcus Dods) was to make the gospel free of expense (adapanon, 1Co 9:18, rare word, here only in N.T., once in inscription at Priene). This was his misthos. It was glorying (kauchēma, to be able to say so as in Act 20:33.).
I have a stewardship intrusted to me (oikonomian pepisteumai). Perfect passive indicative with the accusative retained. I have been intrusted with a stewardship and so would go on with my task like any oikonomos (steward) even if akōn (unwilling).