This refers to the neck brace commonly seen on people who have been in an accident, and sustained injuries to their neck or spinal cord. It's called a dollar collar because
when you get one this way, someone
else is typically paying the medical bills, and some extra, and you sit around and "recuperate".
When you see a (normally black) person
Steve: Did you see that SUV pull out in front of that bus?!
Dave: Man that stupid nigger
must be tryin' to get a damn dollar collar.