What type of gold is relied upon to determine the zakat threshold when gold karats and prices vary? And is the increase in wealth subject to zakat before a full year has passed on the original principal amount of the threshold?

The standard gold purity used to determine the zakat nisab is 24-karat, as it is pure gold. To determine if cash has reached the nisab, it is valued against 85 grams of 24-karat gold.

If the additional two thousand euros are a profit resulting from the five thousand, then the entire seven thousand (principal and profit) are subject to zakat. However, if the two thousand are new, independent funds, they would have their own separate hawl (zakat year), or they can be be zakat-ed with the five thousand for ease.