Of mine own will (εκων)

--not of mine own will (ακων). Both common adjectives, but only here in N.T. save εκων, also in Romans 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 on Matthew 6:1 for μισθος); but the only reward that he had for his willing work (Marcus Dods) was to make the gospel free of expense (αδαπανον, verse 1. Corinthians 9:18, rare word, here only in N.T., once in inscription at Priene). This was his μισθος. It was glorying (καυχημα, to be able to say so as in Acts 20:33).

I have a stewardship intrusted to me (οικονομιαν πεπιστευμα). Perfect passive indicative with the accusative retained. I have been intrusted with a stewardship and so would go on with my task like any οικονομος (steward) even if ακων (unwilling).