How is zakat calculated on foreign currencies (dollars) from the date of purchase, and how is their value assessed?

For zakat (definition) to be obligatory on money (gold, silver, and contemporary currencies), it must reach the nisab (definition) (minimum threshold) and a full year must pass on it. If a Muslim buys currency with money or trade goods, the hawl (one-year period) for the currency is considered the hawl of what it was purchased with. However, if it was purchased with something other than money or trade goods (such as real estate or furniture), or if it was acquired through inheritance or a gift, then a new hawl begins for it from the date of its acquisition if it reached the nisab at that time; otherwise, the hawl begins from the moment it reaches the nisab.