Am I Being Extorted By My Former Employer?

I recently resigned from my collections job due to hostile work environment. For the sake of my mental health I had to leave for many different reasons (bullying, inappropriate behavior, SH)

(Less than 50 employees, I never had an employment contract or signed anything else legal for the company other than my W2 and direct deposit form.)

Cut to a week later I receive an email from my former employer demanding I pay him $10,000+ back for “unauthorized reimbursements”. Otherwise he will be coming after me for “embezzlement”.

From the beginning of my employment I received a standard business expense reimbursements of $350-$500 monthly for costs relating to working remotely (internet, utilities, office space etc.) This was negotiated as part of my salary with my direct supervisor who had full authorization to approve a salary agreement with me.
Several other employees also received standard reimbursements for similar business expenses relating to their positions. Some more than $1000 monthly. For over 3 years of employment there was never any issues or were anyone required to provide receipts. The company owner always had full access to payroll reports and actually constantly reviewed them. Nothing about any reimbursement was ever hidden. All reimbursements were authorized to my knowledge and understanding.

Then I resigned and now I have a threat with a deadline for payment. The company owner has also withheld my final check. Is this extortion? Retaliation for my reason for resigning? I am honestly not sure what steps to take.

Location: Arizona