5 C.F.R. § 591.236
When do COLA and post differential payments end?
Subject to § 591.237(a), agencies stop paying an employee a COLA or post differential on—
(a) Separation,
(b) The effective date of assignment or transfer to a new official worksite outside the COLA or post differential area, or
(c) In the case of an employee on detail to temporary duty in a post differential area, the ending date of the detail.