Hi Ian,
Try this. Go to the field on the form that is rounding up your currency amount. Right click on the field, and then click on properties. Check the Decimal Places property. If it is set to 0 then that is probably the problem. Set Decimal Places to either Auto, or a specific number such as 2.
I just tested this theory, and with Auto my total was $103.97, and when I set Decimal Places to 0 I got $104.
Try this. I hope that it works.