5 CFR §591.237

1. (a)An employee on leave or travel may receive a COLA or post differential only if the agency anticipates that the employee will return to duty in the area. Exceptions: If the employee does not return to duty in the area, the agency may still pay a COLA and/or a post differential for the period of leave or travel, subject to paragraph (b) of this section, if the agency determines that—
   1. (1)It is in the public interest not to return the employee to the official worksite, or
   2. (2)The employee will not return because of compelling personal reasons or circumstances over which the employee has no control.
2. (b)Post differentials. Agencies may pay a post differential to an employee only during the employee's first 42 consecutive calendar days of absence from the post differential area.