If an employer actually sets aside funds to cover the nonqualified deferred compensation obligation that accrues, those funds belong to the employer and the employer can do whatever it wishes with the funds, subject to any contractual (including trust agreement) terms about disposition of the funds. If you mean "can be prohibited" against its will, no. If means other than an appropriate grantor trust are employed to protect or secure the set-aside assets for the benefit of the particpant, the benefit may become taxable.