29 C.F.R. § 553.26 – Cash overtime payments.

(a) Overtime compensation due under section 7 may be paid in cash at the employer's option, in lieu of providing compensatory time off under section 7(o) of the Act in any workweek or work period. The FLSA does not prohibit an employer from freely substituting cash, in whole or part, for compensatory time off; and overtime payment in cash would not affect subsequent granting of compensatory time off in future workweeks or work periods. (See § 553.23(a)(2).)
(b) The principles for computing cash overtime pay are contained in 29 CFR part 778. Cash overtime compensation must be paid at a rate not less than one and one-half times the regular rate at which the employee is actually paid. (See 29 CFR 778.107.)
(c) In a workweek or work period during which an employee works hours which are overtime hours under FLSA and for which cash overtime payment will be made, and the employee also takes compensatory time off, the payment for such time off may be excluded from the regular rate of pay under section 7(e)(2) of the Act. Section 7(e)(2) provides that the regular rate shall not be deemed to include
. . . payments made for occasional periods when no work is performed due to vacation, holiday, . . . or other similar cause.
As explained in 29 CFR 778.218(d), the term “other similar cause” refers to payments made for periods of absence due to factors like holidays, vacations, illness, and so forth. Payments made to an employee for periods of absence due to the use of accrued compensatory time are considered to be the type of payments in this “other similar cause” category.