By Sedara Oluwadara   Cristiano Ronaldo wants Manchester United to let him leave the club if they receive an appropriate offer for him this summer.   The Portugal forward, 37, returned to Old Trafford from Juventus last summer.     However, while he was United's top scorer last season - and third in the Premier League - the overall campaign was seen as a big disappointment.   United finished sixth in the Premier League so missed out on Champions League qualification.   That means Ronaldo, who has a year left on his United contract plus an optional year, faces