All livestock purchased by a packer in cash sales, and all inventories of, or receivables or proceeds from meat, meat food products, or livestock products derived therefrom, shall be held by such packer in trust for the benefit of all unpaid cash sellers of such livestock until full payment has been received by such unpaid sellers: Provided, That any packer whose average annual purchases do not exceed $500,000 will be exempt from the provisions of this section. Payment shall not be considered to have been made if the seller receives a payment instrument which is dishonored: Provided, That the unpaid seller shall lose the benefit of such trust if, in the event that a payment instrument has not been received, within thirty days of the final date for making a payment under section
228b of this title, or within fifteen business days after the seller has received notice that the payment instrument promptly presented for payment has been dishonored, the seller has not preserved his trust under this subsection. The trust shall be preserved by giving written notice to the packer and by filing such notice with the Secretary.