Use Decimal instead.

Even better is to use a data structure that also encompasses a particular currency along with a Decimal amount.

I highly recommend Flight School's book Guide to Swift Numbers, which has an entire chapter on this topic. They have also published a Money package that incorporates all of their advice from that book.

Or if that's too much for your needs, they suggest a simple struct like this:

struct Money {
   enum Currency: String {
      case USD, EUR, GBP, CNY // supported currencies here
   }

   var amount: Decimal
   var currency: Currency
}