The authority is coming from the definition of compensation used. The fact that 3401a is the definition of compensation for the plan, it means that unless you can run the gift card through for payroll processing, which you can't as it is a gift card, then you can't withhold at the source which means you can't set up payroll deductions for 401k, for insurance, for taxes, etc. The only option is to "add it on" to the year end payroll report as an adjustment at it face value of $500 for the employee to pay taxes on.
The question was can they deferring on it as it is added back to payroll, and that answer is NO.
But for the denominator purposes of determining testing compensation, since fringe benefits are not excluded from the base definition of compensation, the cash value of the gift card is in my compensation figure reported for plan testing. The individual just could not defer on it so if he/she was at 5% 401k amount, at year end, they will be slightly less than that because testing compensation will now include the $500 gift card.