It was an inevitable move, United have had a hard on for Sancho for some time, they wanted a right winger and they tend to either pay for their man or go for someone else expensive. Vanity signings are required at United almost as much as Madrid, but they don't have the same pulling power because of recent years. They still have the money, and spend it shoddily.
Dortmund could have had more money off United last year for him but obviously they wanted to keep him for another season for their fight to finish second.
He's a good player for sure but always seems overrated to me. Dortmund suits attacking players and so does the league but then United of the last couple of years under Solskjær also lean in this direction, with mixed success in a league where teams increasingly get success by countering them.