Apple Business Manager. How do we revoke a code or license? We are not using a MDM
The ordinary way to do what you want is to use MDM.

If you do not want your client to setup MDM, then the only way to do this is by adding a login system to your app. This way you can have employees of your customer install the app via the App Store using redemption codes, and then be able to stop them from using the app by invalidating their login.

To make matters easier, you could implement the initial login in the form of a link for the employee to follow (similar to the link used to redeem the app), it could be a QR-code to scan or a simple username/password.