Both are used.
The plural arises normally Some examples from the web showing both American and British use,
Five billion dollars have been appropriated for a two-ocean navy (American writer 1940)
Over five billion dollars have been scheduled to divest from universities since 1985 (American 1990)
five billion pounds have been spent seeking the answer (British 2014 )
However, it is sometimes treated a singular:
The Program will end on January 1, 2014, or whenever the five billion dollars has been spent, whichever occurs first. source
Fifty-five billion dollars has been invested since 2009.source
The difference is whether the money is regarded as a singular thing, the price of a singular object, measured by the unit "dollars". Or treating the dollar as a plural noun.
And so if the money is being spent on a single item I would be more likely to use a singular verb.