Is Zakat to be paid on only $10,000 if an additional $5,000 was not possessed at the start of the Hawl (lunar year), or on the full $15,000?

Zakat is obligatory on the amount that has completed a full year (hawl), which is ten thousand dollars, unless the additional amount (five thousand) is profit from the original amount, in which case zakat becomes obligatory on the entire fifteen thousand dollars.