How do I override position synchronization at the attribute level?

If you want to capture a different value for one attribute only, leave that attribute blank at the position

Let's say the location attribute is synchronized from the position but except in a few cases it doesn't matter into which location the person is hired, then you can leave the location field blank at the position. Since blank values aren't synchronized, the location will be editable in the assignment enabling you to select a value manually.