Is the zakat amount to be returned as is to the man who paid zakat after selling the gold piece, or should a proportion be calculated to determine how much the value of the zakat money has increased, knowing that the value of the gold piece has tripled over the past two years?

Zakat is not permissible except for the eight categories mentioned in the Qur'an, and it is not permissible to give it to a rich person or a strong, earning individual. Whoever takes zakat while not being entitled to it must return it along with any increase that has occurred to it. If it is possible to return it to its payer, that is obligatory. If not, it is obligatory to distribute it to the deserving poor and needy.