In my master branch, approx. 30 commits back, i did improperly merged a commit of my colleague.
I did not realise that in that time, but he came back today that when he was pulling some of the code i did in the meantime, it broke his build since i have not merged that commit back then correctly.
I am using git on mac, and managing the repository with SourceTree.
What should I do, how can i remedy this?
Well, I would suggest doing
git checkout bad-merge-commit, make the changes necessary, commit,git checkout HEAD, merge and make sure this time the merge is correct. If you need all the in between commits to be correct then you can probably do some sort of rebase of those commits onto the fixed merge.