BalancedPayment shouldn't store amount as float

[jbafford]

It is dangerous to store and manipulate currency values as floats, since floats with decimal values can be non-exact, and unexpected errors can creep into calculations.

It would be preferable to store values either using the decimal type (which can exactly represent decimal values), or preferably as an integer (since PHP doesn't do decimal math, and that is what the Balanced API expects).