Can I negotiate a severance package if I am being forced to report to a manager who made slanderous statements about me during my leave of absence?

While I was on my leave of absence for a medical disability, my manager told my colleagues she was "suspicious" of my leave of absence since I seemed fine to her and that she felt it was due to stress. A colleageu and she had a conversation where it was stated that I am taking advantage of my company's leave of absence policy even though I have provided medical certification from my medical provider. I requested not to report to this manager due to the damage that was caused by her unfalse statements but now I am being forced to continue reporting into her when I get back from my leave. I feel pressured to quit since I do not think it would be reasonable for me to continue reporting to her after this incident and it would be difficult for me to continue working with her. Can I negotiate a statement from my manager guaranteeing that she will not make any more statements regarding this situation to protect my integrity as a professional? I plan on quitting because of this situation but can I negotiate a severance so I can keep my unemployment benefits? What are my options? Are they in violation of FMLA or do her statements fall under defamation in the workplace? What s my company’s responsibility in this situation? Have my rights as an employee been violated in this situation?