Sending fake checks through the mail is a crime so contact the US Postal Service Inspectors:
https://postalinspectors.uspis.gov/forms/MailFraudComplaint.aspx
https://www.usps.com/
This is a scam. Guy claiming to hire you as a mystery shopper and then sends a fraud cheque trying to get you to use your own money while waiting for a fake cheque to cash.
Sending fake checks through the mail is a crime so contact the US Postal Service Inspectors:
https://postalinspectors.uspis.gov/forms/MailFraudComplaint.aspx
https://www.usps.com/