You have to evaluate your goods at the completion of the Hawl
I am a merchant. My merchandise consists of items of varied prices. When a Hawl (one lunar year calculated from the time a property reaches the minimum amount upon which Zakah is due) passes, I evaluate the present goods, price according to the current market price and pay their Zakah (obligatory charity). Sometimes, I evaluate the merchandise as more than its value.
One of the learners told me that paying more is like paying less. Let me explain to Your Eminence some of (Part No. 8; Page No. 87
Your evaluation of your goods when a Hawl passes is the Wajib (obligatory) on you as a Muslim. You have to evaluate your goods at the completion of the Hawl regardless of the price of purchase and pay 2.5% of its current market value at the time of payment. What you pay above this amount will be regarded as Sadaqah (voluntary charity) and you will be rewarded for it. If you do not pay that extra amount, there will be no sin on you.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.