Valdes, who has been stuck in limbo between MLS - which owns his contract - and Argentina's San Lorenzo, has been the subject of intense speculation following Colombia's strong World Cup performance. He spent the 2011 and 2012 seasons with the Union before returning to Colombia on loan with Santa Fe in 2013 and then moving to San Lorenzo at the start of 2014.
His strong performances for all three clubs eventually secured him a spot in Jose Pekerman's World Cup squad. Valdes came off the bench once for Los Cafeteros, helping lock down a group stage win over Japan en route to a quarterfinals appearance.
San Lorenzo, who face Bolivar of Bolivia in the first leg of the Copa Libertadores semifinals on Tuesday, have accused Valdes of a "lack of commitment." In a since-deleted tweet, Valdes responded that he was owed five months of wages and would not report to the club.
Philadelphia, meanwhile, have struggled in defense all year, having posted just three shutouts and using a rotating cast of central defenders while conceding 34 goals, second-worst in the league.
The Union declined comment to MLSsoccer.com on the report.