We paid 73m for a player who was 2nd only to Mbappe with being the most productive player before aged 20 in any of the big leagues over the past 30+ years. He's a generational prospect as a creator... And he has the numbers and analytics to back that up in actual professional football, in a top 5 league. He doesn't need to have Rashford level pace to be a top player, he's plenty quick enough.
Amad was signed on the back of less than an hour of professional football, and has proceed to play basically no minutes in the year he was at the club, and then was a scarcely used sub in the Scottish League. He is of course talented, but physically he is lacking so much that he isn't even getting minutes in the Scottish League. Sancho is only 2 years older than Amad. 2 years ago, Sancho was Dortmund's best player and getting 20 goals and 20 assists in the season. There's a bit of a difference in levels there.