Is zakat obligatory on the seller of crops that have not fully grown and were sold as fodder, or on the buyer who found among them a portion of crops suitable for harvest and reaching the نصاب (minimum threshold)?

Zakat on grains becomes obligatory for the owner of the crop when the grains harden. If the crop is sold before its grains harden, then the zakat is due from the buyer. But if it is sold after its grains have hardened, then the zakat is due from the seller.