You may use the Units & Quantities metric conversions.
The quantity system understands metric prefixes, for example "Centi" and "Mega". Therefore you can use TargetUnits if you have Quantity objects for it to convert.
A billion in metric prefix is a "Giga". Either the unitless quantity "Unities" or a specific currency like "USDollars" can be used.
Let
aum = <|"Bridgewater AUM" -> 1 10^11, "Softbank Vision Fund" -> 1 10^11|>;
Then with "Unities"
BarChart[Quantity[aum, "Unities"],
Frame -> True,
PlotLabel -> "Assets Under Management",
FrameLabel -> {Automatic, "Billions $"},
ChartLabels -> Automatic,
ImageSize -> 700,
TargetUnits -> "Giga"]
and with currency "USDollars"
BarChart[Quantity[aum, "USDollars"],
Frame -> True,
PlotLabel -> "Assets Under Management",
FrameLabel -> {Automatic, "Billions $"},
ChartLabels -> Automatic,
ImageSize -> 700,
TargetUnits -> "Giga*USDollars"]
Both produce the following chart.

Hope this helps.
PS: You can also us syntax like "Giga" "USDollars" or "Giga"*"USDollars" instead of all in on string.