Of course. People play up how great some clubs are at selling players and how terrible we are, but the truth is that the market pretty much runs itself.
If there were 4-5 clubs willing to pay Nani over £100k a week and a fee of over £20m then we'd have sold him for over £20m. If there is only one club who will pay up to £6m, then there isn't much the club can do. We could play hardball for an extra £2m, but that risks them pulling out and him potentially costing us another £5m in wages next season. Naturally there are times to play hardball (De Gea) and times where the gamble isn't worth the risk. It's a smart decision by the club.
Likewise people say Chelsea are great at receiving fee's, which isn't really true. What Chelsea are good at is keeping their fringe players on lowish salaries (as well as putting them in the shop window and selling when demand is higher). This lower salary firstly increases the amount of clubs that could get involved in a bidding war and secondly once 4-5 clubs are interested naturally the cost gets driven up. The likes of Bertrand, Lukaku & Salah were all on around £35k per week when sold (or about to be), which even with a decent pay rise would have their salary at around £12m over 5 years, compared to the £25m Nani is on. That £13m differential is the difference between a £19m fee received and a £6m fee received. That's before you take into consideration the bidding war that would take place if dozens more clubs who probably wanted to sign him could afford his salary.
You look at the fees paid for our lower earners and we do quite well. Bebe for £2.5m, Buttner for £4.5m, Keane for £3m, Zaha for £6m. That's all pretty solid business.