My employer reduced the tax withheld in W-2c after I repaid the gross of overpaymant

My employer asked me to repay gross overpaymant of $2500 ($2000 net + $500 taxes withheld   for november 2016)  in march 2017,   I repaid it and got W-2C. in W-2C my employer reduced the tax withheld  also along with wages. Is it correct? How can I get tax withheld amount back for that overpayment