My Employer Is Threatening To Claw Back My Pto Balance If I Use Sick Leave During My Notice Period.

I work for a nonprofit in Washington State, and have, after being offered a higher paying position working in city government, given my thirty-day notice, so they have enough time to post my position and find my replacement. Our company does not have separate banks of vacation and sick leave, so everything is lumped into PTO, which they have compensated for by giving us one hour for every ten worked. Upon putting in a request to use the remaining week of PTO I have accrued to fill my last week at the company, I was notified tersely by HR that they do not allow that. My supervisor showed me the email she got, which went so far as to say that even using sick time during my notice period would, in their view, forfeit my entire PTO balance.

I have been told that in Washington State, it is illegal to penalize an employee for using paid sick leave at any point during their employment, even during their notice period. What are my options if my employer tries to follow through with this threat?