How is Zakat calculated on money paid two years in advance, with uncertainty about the future status of the money?

For zakat to be obligatory, two conditions must be met: reaching the nisab (minimum threshold) and the passing of a hawl (one lunar year). It is not permissible to prepay zakat before the nisab is reached. However, it is permissible to prepay zakat before the hawl passes, after the nisab has been reached, especially if there is a benefit in doing so. This is indicated by the Prophet Muhammad's (peace be upon him) prepayment of two years' worth of zakat from Al-Abbas's wealth. The method of prepayment involves giving out the zakat for the existing wealth for two years. Then, the wealth should be re-evaluated at the end of each year: if the wealth has increased, the excess amount is subject to zakat; otherwise, there is nothing further due.