Is one who buys land for building for the purpose of saving money and escaping the annual obligatory payment of zakat sinful, and is it permissible to convert saved money into something upon which zakat is not obligatory, and to sell it when needed, to avoid the decrease of money due to zakat?

There is no zakat on land not intended for sale, unless it is sold, its price is received, and a year passes over it.

A Muslim should not sell or exchange his wealth on which zakat is due with the intention of evading it. If he does so after zakat becomes obligatory (completion of the year), then zakat is binding upon him. However, if he does so just before the completion of the year, the majority of jurists (Malikis and Hanbalis) hold that he is obligated to pay zakat, treating him contrary to his intention. While Shafi'is and Hanafis believe that there is no zakat on him, though his action is disliked.

Converting saved money into something on which zakat is not due: If it is with the intention of trading, zakat is not waived. However, if it is with the intention of acquisition or selling when needed for consumption, then no zakat is due on it.

Zakat increases wealth in blessings and growth, confirming the saying of the Prophet (peace be upon him): "Charity does not decrease wealth."