Judgment of the Court (Sixth Chamber) of 29 June 1999 in Case C-256/97 (reference for a preliminary ruling from the Tribunal de Commerce de Bruxelles): relating to Déménagements-Manutention Transport SA (DMT) (Article 92 of the EC Treaty (now, after amendment, Article 87 EC) - Concept of State aid - Payment facilities granted by a public body responsible for collecting employers' and workers' social security contributions)