Manchester United have made a habit of selling young players of late but have made sure to put release clauses into many departures. Breaking through from the Manchester United academy and into the first team is not easy. Kobbie Mainoo may have made it look pretty simple, but that the reality is that very few will make the grade. United have shown a real willingness to sell young players who aren’t particularly close to first-team reckoning. The key is that United keep insetting key clauses into those deals, whether it’s a buy-back clause or a sell-on clause. That allows United to still have s…