I don't know if this has been said, but if I'm issuing a fine and I see someone volunteering to pay half of it, I simply double the penalty so the original offender is still paying the right amount.
I could be wrong, but I believe the league has a schedule for fines based on offense and number of times said player has been punished before. If so, I'm sure it's also part of the CBA and the union would fight them doubling a fine.