Purchase from Farmers with Green Vouchers - Valid?

One of my clients was engaged in the business of trading in Pulses. The person is directly procuring the pulses from farmers. The purchase is not being supported by any documents like Bill of Supply or Tax Invoice. Recently my client is producing some local available self made bills named Green Vouchers along with Aadhar of Farmer. My question here is, does this green voucher has any validity? The transaction is genuine and making the payment through bank. What are the correct documents to be kept with us in the regard. How can we safeguard ourself in these type of transactions.

Thanks in advance.