Wa `alaykum salam wa rahmatullah
Apologies for the confusion.
Okay you said that $15,000. And this has reached the level of Nisab. The Nisab for currency is based on the Nisab value of Gold or silver (lower of the two), which is 85 grams for Gold and 595 grams for Silver.
So let's say that the value of 595 grams of Silver is $550 and the value of 85 grams of Gold is $2500. We take Silver (since it is the lower of the two) and once you reach $550 or over, and you fulfill the other conditions of Zakah (i.e. your money did not drop below Nisab level for a whole year and you had complete ownership of it)- then zakah is due on you at the end of the Lunar year.
If it so happens that you based the nisab value of your currency on the Nisab value of Silver, then you add that 30 grams of Silver along with your $15,000 and NOT the gold. If it happens that you based your nisab value of your currency on the nisab value of Gold, then you add 20 grams of gold along with your £15,000 and NOT the silver.
So, based on the example I gave (where the Nisab value of Silver is lower), then you will give 2.5% of ($15,000 + 30 grams of Silver).