The European Commission could open a legal case against Germany over a ruling by the country’s constitutional court that the European Central Bank had overstepped its mandate with bond purchases, the EU executive arm said on Sunday.The German court in Karlsruhe last Tuesday gave the ECB three months to justify its flagship euro zone stimulus scheme or said the Bundesbank might have to quit it.In response, the European Union’s top court - which had previously gave its green light to the ECB scheme - and the European Commission have said that EU law holds precedence over national regulations. They added that the European Court of Justice’s rulings were binding for courts in the 27 member states of the bloc.