Is it legal for an employer to withhold my paycheck for not signing a resignition (sp?) letter?

My Manager told me I was taken off the schedule and fired for not sending the District Manager an email telling her what time and day I wanted to have a disciplinary meeting with her. Now, they are saying I quit because I didn't send the email which is not true, I just didn't send the email. They are trying to get me to sign a letter of resignition and also told me that if I sign the letter, I can still get my June commission. Is it legal to withold my last paycheck if I don't sign the letter? What are my rights? I live in Florida. Thanks