Marcuse’s loyalty to the Institute exceeded Neumann’s despite taking outside work.

Evidence

Less self-confident than Neumann, and considerably more submissive towards Horkheimer, Marcuse remained more closely attached to the Institute than his friend. In Marcuse, the Institute’s directors had what Horkheimer had hoped for in vain from Neumann: someone who was no longer a financial burden to the Institute, but who continued `to feel as one of our group’. — Rolf Wiggershaus, The Frankfurt School, 1252