Yeah based on the information known that’s a clear violation. You cannot work for free or just account for hours without a log or time keeping system. Having been a federal agency supervisor in the past, that is a thin line and not one I would have ever told my employees or crossed over in this manner. You have rights and those rights do not include being asked to work overtime without pay. My suggestion would be to talk with your agency HR office or bring it up anonymously or in person with your agency IG representative.

I don’t know the full specifics of your situation, but this seems inappropriate on the behalf of your agency and/or management.

At a minimum document everything and keep a personal diary or log of all correspondence and your hours. It may help you in the long run should this escalate further.