Is it permissible for a merchant who is unable to itemize his goods to calculate zakat, to estimate the zakat amount based on last year's zakat with an increase, until doubt is removed?

If a merchant deliberately delays paying Zakat after a full year has passed, he sins and must repent and hasten to pay it. However, if he is unable to calculate the value of his goods, he should pay Zakat on his wealth now based on an estimate, and if an excess is found beyond what he paid, it will be considered charity for him.