You shouldn't leave it like that. I don't think they will trace you to Germany (except perhaps if they could put it on your credit file somehow or they take it as a criminal offence to put you on the 'Schengen criminal list' ) but you might one day have to get back to Brussels and then you would have to pay it back with all the additional charges they will put on - and believe me, that might mean increasing your negative balance manyfold !