3Subject as aforesaid, where a relevant vehicle was sold under a purchase made on or before budget day and was delivered under the purchase on or after the first day of May, nineteen hundred and fifty, but before the date of the passing of this Act, and the seller has recovered from the buyer, as an addition to the price, any sum fixed by reference to his prospective liability by virtue of this Act to purchase tax in respect of the vehicle, the buyer shall be entitled to recover that sum from the seller as money received by him for the use of the buyer.