If you want to take the expense, then you would have to consider the reward as other income.
Publication 334 page 27:
Trade discounts. These are reductions from list or catalog prices and usually are not written into the invoice or charged to the customer. Do not enter these discounts on your books of account. Instead, use only the net amount as the cost of the merchandise purchased. For more information, see Trade discounts in chapter 6.
For IRS references,
Publication 334 Tax Guide for Small Business
Publication 535 Business Expenses
