Is it permissible for someone who paid more zakat on trade goods than was due from him to count the excess toward the zakat of the coming year?

It is permissible to advance Zakat. If someone pays the Zakat on their wealth and then it becomes clear to them that the amount paid is more than what is due, they are not allowed to count it towards the Zakat of the following year unless they had the intention of advancing it. If they did not intend to advance it, then it is considered a voluntary charity. So, if someone thought their wealth was one thousand and paid its Zakat, then it became clear that their wealth was five hundred, if they had intended to advance it, the amount paid would be sufficient for two years. However, if they did not intend to advance it, the excess amount cannot be counted towards the Z Zakat of the following year, because they did not intend it for that purpose, and it becomes a charity by which one seeks nearness to Allah.