The handling of decimal numbers from HTTP requests in the FundraisingFrontend is inconsistent:

• donation/new expects English-formatted numbers (using a locale-aware parser) and will default to 0 if the number can't be parsed.
• donation/add expects German-formatted numbers (using a locale-aware parser) and will default to 0 if the number can't be parsed.
• apply-for-membership expects German-formatted numbers (using simple string-replacement of comma to dot) and has no default.
• validate-amount expects an integer (Euro Cents)
• validate-fee can handle both English and German-formatted numbers (using simple string-replacement from comma to dot)

The best way to unify the situation is to get rid of the existing locale-aware AmountParser and standardize on expecting integer amounts in the controller parameters, which represent EuroCents. This will avoid rounding errors. For this solution, the client-side code, both in FundraisingFrontend and in the banners needs to be changed - the browser must still display decimal numbers in the locale of the user, but the value that gets sent to the server must be an EuroCent integer.

Conversely, all amounts in JSON data (provided for templating or the skin) should also be integers representing EuroCent and the frontend code (takes care of formatting it in the right locale). We must also get rid of the AmountFormatter PHP class. While working on the ticket you can take the usage of the class as an indicator that the amount formatting code needs to be changed.

Note: The estimation is made under the assumption that there will only be one skin to be changed.