I recently quit a job where I was forced to pay back a portion of relocation expenses the company gave. This money was taxed as income. What will I have to do to reclaim the taxes I paid on that money? The company made me reimburse them for the full amount, before taxes.
If you return the money in the same tax year - it will not appear on your W2 and you will not be taxed on it. Whatever was withheld - you'll get it refunded when you file your annual tax return.
If you return it in a different tax year - it becomes a miscellaneous deduction reported on your Schedule A. If the amount is less than $3000 - then this deduction is subject to the 2% AGI threshold, if the amount is more than that - it is not subject to threshold.
Bottom line, you're probably going to lose money, unless you're already itemizing and the amount is above $3K.
There's also a credit that you can take instead of deduction. See publication 525 for details.