If you comment out
scrollContent.animate({"margin-left" : margin+"px" }, 300);
you'll notice that when clicking on slide bar scrollContent still moves to a new position. This means that calling animate afterwards will do nothing since margin-left has already changed at some other point of your code. So you need to check whether dragging took place or not before changing margin-left.
A rough example can be found here (look for //EDIT in two places), http://jsfiddle.net/HXgwa/13/ I have not checked your code just added a condition related to dragging before your code changing margin-left.