Fiqh-us-Sunnah Volume 3, Zakaat and Fasting, Fiqh 3.057A.

It is not permissible to pay the value instead of the item itself, except in the case of non-existence, for zakah is an act of worship which can only be fulfilled according to the specified manner, with the rich sharing their wealth with the poor

Mu’azh reported that the Prophet, upon whom be peace, sent him to Yemen and told him: “Take grain from grain, sheep from sheep, camels from camels, and cows from cows.” This hadith is narrated by Abu Dawud, Ibn Majah, al-Baihaqi, and al-Hakim. It should be noted that there is an interruption in the chain of this hadith, since ‘Ata’ did not hear it from Mu’azh.

Disapproving of substitution, ash-Shaukani says: “The truth of the matter is that zakah is obligatory on the item itself and should not be substituted for its value except where there is a valid excuse.”

Abu Hanifah permits the acceptance of the value whether the individual owing could pay it in the items itself or not because zakah is the right of the poor, and he believed that it made no difference whether it was paid in the item or in something else of equal value. AlBukhari reports, with a firm statement, that Mu’azh asked the people of Yemen to give him either goods or clothes of silk or garments as zakah instead of barley and corn because it was more convenient for them. The companions of the Prophet, upon whom be peace, were also given the choice in Madinah.