Employer Withheld My Entire Final Paycheck. Is This Legal In Washington State?

I recently was let go from my job in Washington State, and my employer withheld my entire final paycheck of $1,200. They claimed it was to cover an $800 debt I owed them for services I used for my pet. However, I had a written payment agreement with them where I would pay $100 per month toward the debt. I even made the November payment before I left, and they had agreed to these terms in writing.

I reached out to my employer in writing to ask about this, and their response was simply that I owed them money, so they deducted it from my final paycheck. They withheld:
Overtime pay I earned during my last pay period.
My accrued sick and PTO, which they paid out as part of the final paycheck but kept in full.

The employee handbook does not support anything about withholding final pay for debts upon termination, and I never agreed to this deduction. I’ve already filed a claim with Washington Labor & Industries (L&I), but I feel like this situation is extremely unfair and possibly illegal.

Has anyone dealt with something like this in Washington? Is it lawful for them to withhold my entire paycheck, including overtime and PTO, even though we had a payment plan in place? Also, any advice on next steps while I wait for L&I to investigate?

I just want the money I earned.