Is it permissible to retain Zakat money to make it a large and useful sum, while not currently benefiting from the retained amount?

Whoever has wealth on which Zakat is due must disburse it immediately, because an absolute command implies immediacy, and because it is a right that must be given to the deserving recipients from the eight categories mentioned in the Quran. It is not permissible to delay Zakat except if there is harm in disbursing it at its due time, or to hand it over to someone more deserving with a slight delay, or due to the absence of deserving recipients. It is not permissible to delay it to benefit from the retained amount.