Here's a few reasons why Levy would reject 40 million pounds
1) Arsenal are floundering and Liverpool are still gelling. If Spurs settle down, get past Modric unhappiness (a biggish if), then they have an opportunity to challenge for 4th again.
2) Selling your biggest player unsettles the other players.
3) Selling your biggest player also invites teams to come looking at your other players. The sum of these last two points could potentially lead to vd Vaart and Bale angling to leave in the next 2 windows.
40m pounds is a lot of money, but if you don't have competent replacements in mind, then it's 40 million utterly useless pounds