29 C.F.R. § 5.26

(a) Requirements. The following requirements apply to any fringe benefit contributions made to a trustee or to a third person pursuant to a fund, plan, or program:

(1) Such contributions must be made irrevocably;

(2) The trustee or third person may not be affiliated with the contractor or subcontractor;

(3) A trustee must adhere to any fiduciary responsibilities applicable under law; and

(4) The trust or fund must not permit the contractor or subcontractor to recapture any of the contributions paid in or any way divert the funds to its own use or benefit.

(b) Excess payments. Notwithstanding the above, a contractor or subcontractor may recover sums which it had paid to a trustee or third person in excess of the contributions actually called for by the plan, such as excess payments made in error or in order to cover the estimated cost of contributions at a time when the exact amount of the necessary contributions is not yet known. For example, a benefit plan may provide for definite insurance benefits for employees in the event of contingencies such as death, sickness, or accident, with the cost of such definite benefits borne by the contractor or subcontractor. In such a case, if the insurance company returns the amount that the contractor or subcontractor paid in excess of the amount required to provide the benefits, this will not be deemed a recapture or diversion by the employer of contributions made pursuant to the plan. (See Report of the Senate Committee on Labor and Public Welfare, S. Rep. No. 963, 88th Cong., 2d Sess., p. 5.)