Emigration is the act of leaving one's home country. Immigration is the act of arriving in a new country. There are lots of databases with ship's lists or even immigration documents, but very few that track those leaving Scotland.
FindMyPast has a list of passengers leaving the UK
The University of Aberdeen has an emigration database http://www.abdn.ac.uk/emigration/
If you are having a hard time tracking when your ancestor left Scotland, give one of these emigration databases a try.