Before the sale there was no zakaah on it, because it was not trade goods. After selling it, the zakaah is zakaah that is due on a debt. In other words, when he receives an instalment, he should pay zakaah on it for that year; when he receives another instalment in the second year he should pay zakaah on it for two years; when the third year has passed, he should pay zakaah for three years, and so on. End quote.
Shaykh Muhammad ibn ‘Uthaymeen (may Allaah have mercy on him).