I don't understand why you can't simply use subtraction, regardless of the decimal truncation in either of the operands.
You're going to end up with some floating-point drift anyway; perhaps it would be a good idea to drop the decimal points and treat everything as integers, then divide by 10n when it's time to display the number to the user. (This is the recommended way of doing currency calculations so floating point error doesn't mess with your cents.)
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