Currently, numbers in the tooltip match the format of the Axis.
If you want to have decimal places in the tooltip, but not on the axis, you seem to be out of luck (Unless you create a different field just for this purpose). This means that having decimal places in the tooltip means you must have the same format in the axis, and decimals have no place in an axis.
So what we get is this:
What we should be able to achieve (without calculated fields that are just duplicates of other fields with different formatting) is this:
This can be especially bothersome when the amount is on the x axis: