QUOTE= (snip) AMOUNT OF DENIED BOARDING COMPENSATION Passengers who are eligible for denied boarding compensation must be offered a payment equal to the sum of the face values of their ticket coupons, with a USD 200.00 maximum. However, if the airline cannot arrange "alternate transportation" (see below) for the passenger, the compensation is doubled to USD 400.00 max.

Based on the above I would guess that, even though the passenger signed for alternate compensation, the passenger has recourse to change his mind and get the cash if the airline did not pro-actively make the offer of a payment in cash in addition to vouchers or tickets etc.