Is it permissible to continue in a side job that conflicts with the terms of the employment contract, and what should be done with the money earned from it?

If the employer stipulates that the employee not work outside official working hours, and the employee agrees to this, and the employer has a valid reason for this stipulation, then the condition must be fulfilled, as Allah Almighty says: "O you who have believed, fulfill [your] contracts." However, if the employer does not have a valid reason for this condition, then it holds no weight. In the event that the condition is binding, if the employee works outside [official hours] without the employer's permission, he is sinful for violating the condition, but what he earns from that work is not forbidden to him.