Is it permissible for me to take payment for 24 training hours, knowing that the actual training was 20 hours, especially since the relevant regulation does not address this?

Contracts must be fulfilled, as Allah Almighty says: ﴿O you who have believed, fulfill [all] contracts,﴾ and the Prophet, peace and blessings be upon him, said: «Muslims are bound by their conditions.» Deceit, lying, and devouring people's property unjustly are forbidden. Therefore, if the agreement was for a 24-hour course, then this must be fulfilled. It is not permissible to give less and write something untrue, for that involves lying, deceit, and devouring people's property unjustly. Thus, the course must be given in full, and if there is a deficiency, the actual number of hours must be disclosed.