There is only one answer, and it comes in three flavours:
a) the name of a currency -
singular:
"The Dollar against the Bitcoin went down by 10 percent."
"The Euro against the Bitcoin went down by 10 percent."
"The Bitcoin against the Pound went up by 10 percent."
b) means of payment -
it depends:
"I spent my last ten dollars on ..."
"Soon crossing 17 Trillion Dollars in Federal Debt."
But:
"I lost 50 Euro."
"This cost me ten quid."
c) amount in spoken language -
singular:
"... costs five dollar fifty"
"... costs five euro fifty"
"... costs five bitcoin fifty"
