It doesn’t have to be the entire €3,000 bonus. How about 600 euros? Employers are allowed to grant their employees tax-free salary supplements and social security benefits in kind. “Compensation in kind” because it must not be a simple salary increase, but a monetary benefit in kind, including goods and services.
There are various possibilities as to what those goods or services the boss is offering you may be. It is important that updated regulations are adhered to.