Toronto, which also re-signed goalkeeper Joe Bendik to a multi-year deal Tuesday, acquired a conditional first-round pick in the 2015 SuperDraft from the Sounders for Frei. The move came a day after Seattle declined the option on veteran Michael Gspurning, a 2012 Goalkeeper of the Year finalist who struggled to find consistency this past season.
Frei, 27, started 81 games for Toronto from 2009 to 2011 but made just one MLS appearance over the past two seasons. He suffered a broken leg early in 2012, then found himself behind Bendik on the depth chart last season.
United, meanwhile, sent an international roster slot for 2014 and 2015 to the Impact for Arnaud, who had served as the club's captain during its first two MLS seasons. Arnaud, 33, figures to help fill the leadership void left in United's locker room by the departure of captain Dwayne De Rosario and the possible exit of midfielder John Thorrington, who is eligible for the re-entry draft.
Arnaud compiled one goal and five assists in 24 games (21 starts) for Montreal last season.