My question involves labor and employment law for the state of: Pennsylvania
I was presented an offer letter which stated that I was to recieve a "Non-Refundable Commission Draw" for the first 90 days of employment. There were no length of employment requirements stated in the offer letter. I asked, and my employer agreed, to extend the Non-Refundable Commission Draw period an additional 30 days. I received a verbal confirmation of this extention from my supervisor, and he did not convey to me any requirements for the additional 30 days. I accepted and resigned my position with my current employer.

On my first day of employment I was asked to sign an "Addendum to the Non-Refundable Commission Draw", which stated that if my employment ended (termination or resignation) prior to 1 year I would be required to pay back the entire non-refundable commission draw.

Is this addendum enforceable?