2 problems with your logic.
1. Resale value: A 27 year old Rose, one of the top LBs in the league, is probably worth around 50M with the insane Prem tax (at 27, he'll have only a little bit of value in 4-5 years as a tiny 31-32 year old fullback reliant on pace. You'r talking about at most 10-15M for a player with that profile).
Martial is already decent and due to his age, you could have him for 4 years and resell him for a profit or no loss at 25 and it's not like he's injury prone, and his ball control and pace aren't going away, so pretty safe investment even if he ends up being a very, very good rather than great player, like Callejon or Pedro instead of Neymar.
2. United don't sell players they don't want to Spurs. If they want Rose and will pay the fee Levy thinks helps Spurs, we'll get him. If not we won't. Just like if Spurs wanted to sign Mahrez from Leicester they wouldn't just include Walker-Peters along with some cash unless they wanted to. That might be better for football, but it's just not how it works. Smaller teams get to ask for money, but they rarely get the most talented young players from the other side in return.