14 CFR § 93.224
Return of slots
July 13, 2020
(a) Whenever a slot is required to be returned under this subpart, the holder must notify the office specified in §93.221(a)(1) in writing of the date after which the slot will not be used.
(b) Slots may be voluntarily returned for use by other operators by notifying the office specified in §93.221(a)(1) in writing.