How can I encode a €-sign to €-sign ?

by an expression?

not sure why you want to encode it for what?

I want to use a string that could have special-caracters like € in a note-attribute of an XML document. This is done in a extension.

When the XML document is serialized by the extension the euro-sign in the note-attribute is replaced by a question-mark (?)

example:

<cbc:Note>Het bedrag van ? 1.015,57 zal omstreeks 14 januari 2016 van uw rekening worden afgeschreven.</cbc:Note>

but the text-value that is passed to the extension was: 

"Het bedrag van € 1.015,57 zal omstreeks 14 januari 2016 van uw rekening worden afgeschreven.".

Please verify the encoding. It should be ISO-8859-1 for that. Additionaly I would recomend you to add a CDATA to your element (<![CDATA[]]>) to prevent any error when loading your XML back again

I find it weird though that the platform seems to use ISO-8859-1 instead of UTF-8 or 16. I'd expect it to be fully Unicode compliant.