Is the money taken by the tenant from the property owner in exchange for vacating the apartment lawful or unlawful, especially if the owner forced him to leave and paid him an amount, even though the tenant would have left it later without compensation?

Leasing is a binding contract for both parties, so it is not permissible for either of them to terminate it without cause. If you rented the apartment for a specific period, you have acquired its usufruct, and the owner does not have the right to force you out. However, if you agree to leave for free, or if you both agree on a sum of money in exchange for vacating it, then there is no objection.