[Ruling on Delaying Zakat, and On What Price It Is Calculated]

Answer—and Allah is the Granter of success: It is not permissible to delay zakat beyond its time except for an excuse, as indicated by His saying, Exalted is He: “And give its due on the day of its harvest.” [al-Anʿām:141].
As for the second part of the question: it is obligatory to appraise the commodity at the prevailing price at the time zakat is paid—whether its price had previously increased or decreased.
The proof for this is that zakat relates to the commodity itself; if the commodity remains, the poor are entitled only to one quarter of one tenth (2.5%) of it, whether the commodity has become cheaper or not.
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1