This is going to turn on what the offer letter says. If the offer really did not expire, you can make an argument that it is paid in full. However, at this point, you would need to file a claim of exemption to stop the wage garnishment. You should have been served with a copy of the notice. You can find the forms at www.courtinfo.ca.gov/forms along with instructions for completing them. You should also have the paperwork reviewed by an attorney to see if you might have any other arguments, including potentially Rosenthal Act violations or FDCPA Violations.