A.T. Robertson Commentary 2 Corinthians 8:3

"For according to their power, I bear witness, yea and beyond their power, [they gave] of their own accord," — 2 Corinthians 8:3 (ASV)

Beyond their power (παρα δυναμιν). "Alongside" with accusative like υπερ δυναμιν in 1:8. Field (Ot. Nov.) quotes Josephus (Ant. iii. 6, 1) for κατα δυναμιν and παρα δυναμιν as here. Few give κατα δυναμιν (according to actual ability). Paul commends this high pressure collection because of the emergency.

Of their own accord (αυθαιρετο). Old verbal adjective (αυτοσ, αιρετος from αιρεομα, to choose), of their own initiative, voluntary. Only here and verse 17 in N.T. Papyri often have εκουσιως κα αυθαιρετως (willingly and voluntarily).