Merchant discounts or coupons are not taxable income.
However, if this was a business trip, you can't take an expense for something that you didn't pay or get refunded. (In a super-rare edge case, if you took the trip on December 29, 2023, and it was a business trip, and you deducted the expense on your 2023 tax return, and you then get a refund on January 3, 2024, then it is a taxable reimbursement of a previous deduction.)
