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Well you can do this,
Whenever the boss sends the authentication number to a particular employee, save that authentication number in firebase under the employee’s details as for eg. – "todaysAuthNo: 123456" , when the employee(user) enters the number and press the go to work button then check whether the number entered by the user matches with the number entered by the boss i.e. check whether it matches with "todaysAuthNo: 123456".
Hence, next time when the boss enters a new authentication number it will get replaced with the old number in "todaysAuthNo" and also the user will need to enter the new authentication number entered by the boss to login.
Source:stackexchange.com