When creating a contribution in the back-end, total amount is incorrect

If the locale of CiviCRM is a country where the decimal separator is a comma (such as ... all of Europe!), entering a contribution with a decimal transaction fee leads to an incorrect total amount.

So for example a contribution of 50 € with a fee of 0,95 € leads to a total of - 45 € (minus 45).

This is due to the Javascript calculating the total assuming that we are with an English locale and deleting all commas in the amounts. So calculating: 50 - 95 = -45