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Who maintains receipt for employee expense reimbursements?
In the normal course of events, you should receive a separate check for the amount of the purchase, and that amount should not be included in your wages as shown on your W-2 statement.  If the amount is included on your paycheck, it should still be listed separately as a non-taxable item, not as part of wages paid.  In other words, the IRS should not even be aware that this money  was paid to you, there is no need to list the amount anywhere on your  income tax return, and if you are paranoid about the matter, staple the stub attached to the reimbursement to a copy of your bank statement showing that you deposited the money into your account and save it in your file of tax papers for the year, just in case the IRS audits you and requires you to document every deposit in your checking account.  The amount is a business expense that is deductible on your employer's tax return, and your employer is also required to keep documentation that the  employee expense reimbursement plan is running as per IRS rules (i.e., the employer is not slipping money to you "under the table" as a reimbursement instead of paying you wages and thus avoiding the employer's share of FICA taxes etc) and that is why your employer needs the store receipt, not a hand-written note from you, to show the IRS if the IRS asks. You said you paid with "your own cash" but in case this was not meant literally and you paid via credit card or debit card or check, then any mileage award, or points, or cash back for credit card use are yours to keep tax-free, and any interest charges  (if you are carrying a revolving balance or paid through your HELOC) or  overdraft or bounced check fees are yours to pay.