It seems like he refused because of her association to Epstein (or perhaps just not being interested), and knew she was probably underaged and coerced despite her own coerced statements.

People are giving Mr. Minsky flak for not refusing the donation, but refusing before the wide publicity of the Epstein scandal probably would have landed him in hot water for some variation of "why did you not take the free money?" or "why are you accusing this upstanding donor of human trafficking and statutory rape?"