Please see this answer from DDollar.
It sounds like you were reimbursed under an Accountable Plan. If that is correct, you do not need to do anything. According to the IRS:
How you treat reimbursements depends on the arrangement you have with your employer.
There are two basic types of reimbursement arrangements—accountable plans and no-naccountable plans. You can tell the type of plan you are reimbursed under by the way the reimbursement is reported on your Form W-2.
To be an accountable plan, your employer's reimbursement arrangement must require you to meet all three of the following rules.
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Your expenses must have a business connection. This means your expenses must be deductible under the rules for qualifying work-related education explained earlier.
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You must adequately account to your employer for your expenses within a reasonable period of time.
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You must return any reimbursement or allowance in excess of the expenses accounted for within a reasonable period of time.
If you are reimbursed under an accountable plan, your employer shouldn't include any reimbursement of income on your Form W-2, box 1.
Non-accountable Plans
Your employer will combine the amount of any reimbursement or other expense allowance paid to you under a non-accountable plan with your wages, salary, or other pay and report the total on your Form W-2, box 1.
You can deduct your expenses regardless of whether they are more than, less than, or equal to your reimbursement.
