Each registrant shall, before the close of the next business day following the day of the purchase of livestock and the determination of the amount of the purchase price, transmit or deliver to the seller, or his duly authorized agent, the full amount of the purchase price, unless otherwise expressly agreed between the parties before the purchase of the livestock. Any such agreement shall be disclosed in the purchaser’s records and on the accountings or other documents issued by the purchaser relating to the transaction.