can overpayment be taken out of severance

Recently laid off and signed a severance agreement. A couple months later the company gets back to me saying that I owe about $4000 in overpayments as I was incorrectly classified as working third shift with a 15% pay differential when I should have been paid 10% differential for 2nd shift. They are withdrawing $500 per severance paycheck until the balance is paid fully. WA state law (WAC 296-126-030) states that overpayments can only be collected if they were irregular mistakes. Even if they were irregular they only have 90 days to collect. Employer coming back saying that only applies for regular paychecks and they have the right to withdraw that from a severance agreement which says that I am responsible for all monies owed. Of course at the signing of the agreement, it was assumed I didn't owe anything and there's nothing on there stating I owed $4000. Do you think the employer is able to withdraw this money from severance checks or would this be prohibited and fall under regular paychecks or future wages which WA state overpayment law mentions. There is nothing on WAC 296-126-030 that specifically mentions severance paychecks. We're dealing with semantics. Thank you.