That sounds like the problem. The {1} is replaced by the system, and my guess is (without looking at it) that leaving it out makes it default to 5€ rather than the amount you specified.
So, as an example:
"Donate {1} to help our site."
The {1} we would replace with the amount, so your phrase translation needs to take into account where you would want to include the amount. An example of translating that in Spanish (using Google Translate) would be:
"Dona {1} para ayudar a nuestro sitio."