If the processor has entered into a contract with two or more producers or consignors, which provides that the returns for any farm product which is handled and sold for the account of such producers or consignors shall be pooled on a definite basis as to grade or quality, or both, during a specific period of time, then the processor shall render a true account of sale which shows the net average pool return on each grade and quality from the sales that are made, which shows in detail all charges in connection with the handling, processing, and selling of such farm product. The processor shall keep a correct record of such sales and charges.