A.T. Robertson Commentary 1 Corinthians 9:18

"What then is my reward? That, when I preach the gospel, I may make the gospel without charge, so as not to use to the full my right in the gospel." — 1 Corinthians 9:18 (ASV)

So as not to use to the full (εις το μη καταχρησασθα). Εις το for purpose with articular infinitive and perfective use of κατα with χρησασθα (first aorist middle infinitive).