Revit seems to make it hard work to work out the task of closing a workset enabled file.

If you are working on a local copy and you make some changes but then decide you dont want to save, Revit wont let you relinquish the borrowed worksets unless you save to central.

Here is the process for getting around it:

Click File > Close
When Revit warns you to Save To Central, choose Don't Relinquish. Revit then asks if you want to save your local file click > No

Now the file is closed: You didn't Save To Central and didn't save the local file but you still have worksets borrowed.

Open the local file again and Save To Central, now go to File > Close or continue working.

There are other methods but this one I have found to be the easiest.