41 CFR § 301-52.16
What must I do with any unused tickets, coupons, or other evidence of refund?
June 9, 2020
You must submit any unused tickets, coupons, or other evidence of refund to your agency in accordance with your agency's procedures.
[FTR Amdt. 70, 63 FR 15969, Apr. 1, 1998; 63 FR 35538, June 30, 1998]