Is zakat due on an apartment intended for trade, then could not be sold at a suitable price? And does the zakat differ if the intention changes to personal use, then gifting it to a son in the future?

As long as you purchased the apartment with the intention of trading and a full Hijri year has passed over it, you must pay zakat on it by assessing its value and giving one-quarter of one-tenth of its value.

If you change your intention to ownership (qinya): - Before a full year has passed: you are not obligated to pay zakat. - After a full year has passed: zakat for the past year is obligatory upon you, but you are not obligated to pay zakat for the future.

The jurists have stipulated that the intention of ownership regarding trade goods nullifies the obligation of zakat on them.