I starting working a file that had a previous plea offer sent to counsel: deferred and a $300 fine for speeding in a construction zone with workers present. Defense cut the check and sent it into our office. The problem is that we can't offer deferred for the offense. See CCP 45.051(f)(1).

I can't imagine a situation where I would exercise discretion to rescind another attorney's offer, even if I thought the offer was terrible. But I don't think there's any discretion here.

I think the solution is to simply call the attorney and explain the issue, and knock $100 off the fine as a gesture of goodwill. But I was wondering if anyone else here has ever had a similar experience.

You've got the answer you need. You can't agree to something that's not allowed by law.