Amounts/Conversions beg for abstraction

[jurajpiar]

There are certain issues in the app where the values don't add up pre and post TX. This indicates an inconsistency within unit conversions and/or general numeric handling.
A layer to handle all numerical value conversions should be designed and used across the app.
@julianmrodri Please, use this issue to outline your findings about the issue.

@julianmrodri @nduchak @farinalvaro Should you have any design ideas, please use this issue to define them.