Hello Remco, Thiago,
So I have found the root cause and the behavior you are encountering is at least 'by design' when it comes to the built in validations.
When you set the 'Currency' local variable to Data Type 'Currency' OutSystems does not allow commas any more. I confirmed this by trying to set the Default Value to 10,000. As you see below it gives me a syntax error about the unexpected ','.

So the ideal way at this point seems to be to set both the Input field and its associated local variable 'Currency' to data type 'Text'. Hope this helps. At least we now know why you were getting the validation message!
Regards
AJ