Whose word is accepted regarding the quantity of minerals taken?

Primary text

The word of the extractor (the one taking the material) is accepted regarding the quantity taken, as he is considered a guarantor (ghārim). If the extractor purifies the material and the amount subject to Zakat equals the required Zakat due, it is accepted. If the remainder exceeds the Zakat due, the excess must be returned unless the owner grants it freely. If the amount is less than the Zakat due, the obligation falls upon the original owner (the one liable for Zakat).