Ah, I think I see the problem. I thought you wanted to go from the child to the master, but you need to go from the master to the child.
In that case, the link in the master should be: mergedProjects/CM/Introduction.htm (if the topic is in the root of the master project.)
Only if you go from the child to the master, the link in the child should be: ../../Index.htm
Kind regards,
Willam