Engaging a debt collector to collect a debt which is still being negotiated by the consumer doesn't violate the Fair Debt Collection Practices Act or California's Rosenthal Fair Debt Collection Practices Act. I don't see any reason why a creditor couldn't send such a debt to a debt collector.
That being said, I would suggest that you continue to negotiate in good faith with the creditor while dealing with the debt collector the same way you would any debt collector. If you don't know how to handle debt collectors, try searching for past questions on this website or asking a new question.
I also suggest that you familiarize yourself with both the Fair Debt Collection Practices Act and the Rosenthal Fair Debt Collection Practices Act. Both laws are fairly easy to understand.
I'm not an attorney. This is not legal advice. You should consult an attorney licensed to practice law in your jurisdiction.