Is Zakat obligatory on the farmer’s entire crop, from which he borrowed a "ardab" of grain, or on what remains after isolating the "ardab" that is owed as debt? And what is the ruling if the crop falls below the "nisab" after isolating this "ardab"?

Scholars have differed regarding whether debt prevents the zakat on crops. Some have considered it a preventative factor, though they are a minority, while others have not considered it a preventative factor, and they are the majority. The preponderant opinion is that of the majority. It is stated in the books of the Shafi'i school that debt does not prevent zakat on crops, fruits, and livestock. Similarly, the Malikis hold that the zakat on tilled land, minerals, and livestock is not waived by debt.