Is it permissible for me to take 1500 Riyals in exchange for assisting in the completion of work for a company whose owner is unaware of my involvement, knowing that my colleague may have obtained illicit gains from this work? And what is my duty towards my colleague and the employer in this situation?

The ruling depends on understanding the nature of the agreement between the employer and your colleague. If the agreement was that your colleague would find someone to do the drawings and would receive a fee for that, then this is permissible, and the money you receive from him is lawful. However, the employer must not care who performs the work, but rather that it is completed according to the required specifications.

However, if the employer has a specific purpose in working with a particular office or engineer, or stipulates that the work must be done outside the Kingdom, then the agreement must be fulfilled. If your colleague is merely an intermediary, then he is an agent for the employer, and he must inform him of the truth of the matter and return any excess money to him.

If your colleague's work is forbidden (haram), then it is not permissible to assist him, and he should be advised to fulfill his trust. As for the calls and follow-ups you made, if you did them with the intention of recovering the cost from your colleague, then you have the right to do so. If it was a voluntary act, then you have no right to claim anything.