Sheikh Sayyed Sabiq, may Allah bless his soul, says in his well-known book, Fiqh-us-Sunnah: “Zakah must be paid immediately at its due time. Deferring the payment of Zakah is prohibited, unless a person, for some valid reason, cannot pay it on time. In such a case, he may wait until he becomes able to pay it. Imam Ahmad and Al-Bukhari narrated that `Uqbah Ibn Al-Harith said: “Once I performed the `Asr prayer with the Prophet (peace and blessings be upon him). When he finished the prayer, he hurriedly went to his house and returned immediately. Noticing the amazed faces, the Prophet said: ‘I left at home a piece of gold which was meant for sadaqah (charity), and I did not want to let it remain a night in my house, so I ordered it to be distributed.’
Thus, in the light of the above-mentioned hadith, it’s clear that, without any tangible and valid reason, it’s not allowed for a person to delay paying Zakah, as long as it’s reached payable amount.