Philemon 14-ἀλλὰ κατὰ ἑκούσιον

Greek (SBLGNT): χωρὶς δὲ τῆς σῆς γνώμης οὐδὲν ἠθέλησα ποιῆσαι, ἵνα μὴ ὡς κατὰ ἀνάγκην τὸ ἀγαθόν σου ᾖ ἀλλὰ κατὰ ἑκούσιον·

"but I preferred to do nothing without your consent in order that your goodness might not be by compulsion but of your own accord" (Philemon 14 ESV).

κατὰ ἑκούσιον only appears in the NT here. 

Stephen H. Levinsohn explains that in Philemon 14b, "the focal constituent ὡς κατὰ ἀνάγκην is preposed in order to set off κατὰ ἑκούσιον to advantage by contrast" since the slave's ceding assent to the apostle Paul is made salient earlier in the verse. 

See Levinsohn, "Analysis of Non-Narrative Texts with particular reference to Paul’s Letter to Philemon in Koine Greek" (SIL International), 2008. Page 33.

For more about κατὰ + the accusative, see Daniel B. Wallace, GGBB, page 377.