Withdrawing a chapter proposal: Is anything else required here? I have been invited to write a chapter with a publisher, so I wrote a proposal for the chapter using the abstract of a write-up (short primer paper) I had. After sending the proposal, I eventually found out that the publisher’s reputation is controversial.

So I gave it a second thought and wrote an email shortly (within two days) indicating I will like to withdraw my proposal. The next day after sending the email, I got an email from the author service manager stating that the editor has accepted my proposal and 'he has no further suggestions for you at this time. Please continue with the writing process' and that after the review of the chapter, I will get an email of definitive acceptance.

Now, I am about to send the short paper to a magazine. Is there anything else required of me with respect to the first publisher to absolve my work from their publication process.
Tell the author service manager again that you are withdrawing your proposal. Use clear language: explicitly say that you're no longer willing to contribute to the book. It sounds like the author service manager didn't realize you're withdrawing and is still expecting a submission from you, and it would be unethical to not correct them.