sign-on bonus

My offer letter stated that my comp plan is base+commision(when met quota). They gave me sign-on bonus for working at least 6 months. I have worked 3 months, they changed the commission plan which is lower than listed on my offer letter. I resigned, HR threaten to sue if i don't agree to refund the sign-on bonus. what should i do?

I would consult with a local employment law attorney about this matter. From what you wrote, you may have a written contract of employment in that offer letter. Or, that offer letter may simply be an invitation to work. So do find an attorney near you via https://attorneypages.com