That which was lacking on your part (τὸ ὑμέτερο ὑστέρημα)
Or the (i.e. my) lack of you. The Greek will bear either rendering. Compare Phi 2:30; 2Co 8:14; 2Co 9:12. The latter is preferable. Edwards, somewhat naively says: “I do not see what could be lacking on the part of the Corinthians which Stephanas and his two friends could supply at Ephesus.”