In the narration of Bakr ibn Muhammad from his father, he said—after his father had asked him about the black dirhams: “When zakat becomes due on two hundred of these dirhams of ours, I deem zakat obligatory on them.” Thus, he adopted the cautious position. He said, “As for the diyah (blood money), I fear for him.” He preferred, with regard to zakat, that one pay from two hundred of these dirhams; but if a man owed a diyah, he should pay full-weight black dirhams. He said, “This is a matter the common people cannot bear.”

The apparent meaning is that he adopted the seven-mithqal standard for zakat, and that the kharaj (land tax) is measured according to the same standard, while for the diyah he required a weight greater than that.

In the narration of al-Marwadhī, he mentioned the small dirhams used in Yemen, each of which weighed two and a half dāniqs. He said, “They should be converted into mithqals. How is zakat to be calculated on these?” Thus, he explicitly stated that every ten of them were to be reckoned as seven mithqals.

There was disagreement concerning why they settled upon this weight. Some scholars stated that, during the time of the Persians, dirhams were minted according to three weights: one dirham weighed one mithqal, or twenty qirats; another weighed ten qirats; and a third weighed twelve qirats. When Islam required their value to be determined for zakat, the average of the three weights was adopted: forty-two qirats in all, yielding fourteen qirats per dirham. Thus, when the Islamic dirhams were minted according to the average of this middle weight among the three weights, it was said of ten of them that they weighed seven mithqals—for that is indeed what they weighed.