It's probably correct. You were paid wages of so much in 2017 and your W-2 should reflect that. When you repaid that amount in 2018 you are, in effect, reducing your taxable wages that will be reported on your W-2 for 2018.
Of course, check with your payroll to be sure. However, there would be no way for them to accurately/legitimately reflect in their 2017 books, something that happened in 2018.
Please feel free to post any additional details or questions in the comment section.
