May my employer recover an overpayment they made to me without my permission?

The deductions permissible under Pennsylvania law are governed by the Pennsylvania Wage Payment and Collection Act.  43 P.S. § 43 P.S. § 260.1 et. seq.  The Act provides that wages are to be paid on a regular pay day in full except for deductions provided by law or as authorized by regulation of the Department of Labor and Industry for the convenience of the employee.  Regulations passed by the Department of Labor do not specifically permit the deduction of anything from wages to collect a prior overpayment.  Any such deduction would have to be authorized by the employee in writing and/or authorized by the Department of Labor and Industry pursuant to 34 PA. Code. § 9.1(13).

This is different from garnishment, which is the process by which creditors obtain a court order permitting the deduction of all or a portion of an employee's wages to satisfy a debt that has been reduced by court order to a judgment.  Without a court order, no garnishment is permissible.