It's been nearly four months since Ursula von der Leyen, the president of the European Commission, and President Trump made a handshake deal to resolve trade tensions between their economies. But details of the trade pact have proven contentious — and they will be up for debate again this week, when American trade officials visit their counterparts in Brussels. Jamieson Greer, the U.S. trade representative, met Maros Sefcovic, the European Union's trade commissioner, on Sunday. Both Mr. Greer and Howard Lutnick, the American Commerce secretary, will attend a meeting of trade ministers in Brussels on Monday.