Is it permissible to change the intention of zakat paid by mistake (due to ignorance of its amount) to sadaqah (charity), and is zakat obligatory upon the owner of the wealth or upon its manager?

Whatever is given in excess of the obligatory zakat amount is a good deed for which its doer will be rewarded. The fundamental principle is that the zakat payer should give the obligatory amount to fulfill their obligation. If they give more, that is better for them, as Allah Almighty says: "And whoever volunteers good – then indeed, Allah is Appreciative and Knowing." The increase may be in the quantity or in the quality. Scholars have differed as to whether the reward for this increase is like the reward for obligatory zakat or like the reward for voluntary charity. Zakat is obligatory on the owner of the wealth, on the principal capital along with their share of the profit, if it reaches the nisab either by itself or by adding it to other money or trade goods. The owner of the wealth is responsible for its zakat, and it is not permissible to give it out without their knowledge. Another person can be appointed as an agent to give out the zakat after obtaining the owner's permission.