His boss gave him the job of following up on some technical drawings, and he managed to save some money on that
The ruling in this case depends on knowing the nature of the agreement that was made between the boss and your colleague. If the agreement between them was that your colleague would look for someone to do these drawings, and he agreed with the boss on the money to be paid for that job, then he was to find someone who would do the job for whatever amount they agreed, then if any of that money is left over, it belongs to him, and if the cost was greater, then he is to pay the extra. This is permissible and there is nothing wrong with you taking the money that your colleague gave you. In this case your colleague is like a contractor who takes on the job, then assigns it to the one who will do it.
But you stated that the agreement was based on the idea that he was going to work with the engineers abroad, and that you told your colleague about local engineers. There is nothing wrong with that, so long as the boss does not care who does the work, rather what he cares about is that the work should be done to the required specifications only.
But if the boss wanted the work to be done with a specific office or engineer, or he specified that the work should be done outside the Kingdom so that company secrets would not be divulged and cause some problems and the like, then it is essential to fulfil what was agreed, and it is not permissible to go against it.
If the agreement between them is that your colleague was just the middleman between the boss and the engineers, and was not to make any profit from this work, then he is acting as the agent of the boss, and the agent does not have the right to make money from the one who appointed him except with his knowledge. In that case, your colleague has to tell the boss what really happened, and that those who are going to do the work are engineers inside the Kingdom, not abroad, and he has to return to him the surplus money.
If the work that your colleague is doing is haraam, in the way mentioned above, it is not permissible for you to help him, either by telling him about engineers or any other kind of help. Rather you should advise him to fulfil the trust and to avoid consuming that which is not permissible for him.
With regard to what you did of phone calls, following up and making appointments, if you did that with the intention of going back to your colleague and asking for the usual payment for such work, then you may do that. But if you did it voluntarily, you do not have the right to ask for anything, but if he gives you something in this case willingly, we do not think that you should take it, unless he has cleared the matter with the boss and he has let him keep the money saved.
And Allah knows best.