As @RomainValeri pointed out in the comments, the git revert command is what you're looking for here.
First, let's visualize the situation:
o---o---o---o---o---X---M
\ /
A----------------
Here, commit A is the one from September 3rd and M is the merge commit from October 21st.
You mentioned that you want to revert just the changes introduced by A from the branch that contains M. You can do so by reverting the merge commit M with:
git revert -m 1 M
where M is a reference pointing to the merge commit.
Now, since a merge commit has 2 or more parents, you also need to tell Git which of the parents it should use as a baseline to determine what changes it should revert. That's what the -m (--mainline) option is for. In this case, we want the target branch (i.e. the branch where A was merged into) to be the baseline, so we say -m 1, a.k.a the first parent.
This will apply the reverse of the differences between the first parent of M (in this example, X) and A in a new commit W:
o---o---o---o---o---X---M---W
\ /
A----------------
It's common to refer to the reverse of a merge commit as W, since it looks like an upside down M.
Keep in mind that reverting a merge commit undoes the changes, but it doesn't undo the history. In other words, Git will still consider the branch that was merged with M to be merged even after W. If you tried to merge it again, Git would tell you that everything is up-to-date.
So, if you want to merge that same branch again in the future, you'll have to revert the revert first. Have a look at this how-to from Git's documentation for details on how to do that.
