A.T. Robertson Commentary 2 Corinthians 8:11

"But now complete the doing also; that as [there was] the readiness to will, so [there may be] the completion also out of your ability." — 2 Corinthians 8:11 (ASV)

The readiness to will (η προθυμια του θελειν). Old word from προθυμος (προ, θυμος), forwardness, eagerness (Acts 17:11). They were quick to pledge.

The completion also (κα το επιτελεσα). The finishing also (articular first aorist active infinitive).

Out of your ability (εκ του εχειν). "Out of the having," literally, and so, "out of what you can give" (verse 12).