United paid £90m for Pogba.We might lose money selling him next summer instead of this one but prices inflate every year so might not lose out as much as you think in terms of the actual fee.
No one ever, ever, ever is spending £100m on him. No chance.
Lukaku is a couple of months younger, counts as a home-grown player (as does Pogba, apparently) and plays a position where the top players cost more. He is slightly better than a 1 in 3 international goalscorer (20 goals in 56 games) and virtually a 1 in 2 prem goalscorer ( 84 goals in 171 appearances for Everton and West Brom - I haven't included the 10 appearances he made for Chelsea).
In the prem, only Ibrahimovic, Kane and Augero are comparable to Lukaku, and there is no chance in hell of any of them signing for us, whether we sell Rom or not. How much do you think Spurs would demand for Kane, if City, United or Chelsea came knocking? £80m minimum, right? And Rom is less injury prone, provides more assists and scores more goals in open play than Kane... despite Kane having the better midfield supplying him.
£100m for Rom is a more reasonable fee than £90m for Pogba. I'm not saying we'll GET £100m, but if we sell him for anything under £70m then the buyer is getting an absolute bargain.