Broadly, Peel would have been willing to sell the site for A LOT of money, reflecting their investment in the site and of course the profit they could realise from developing it. Obviously we don't know what that figure might have been but you'd have to suspect a few hundred million pounds. If they really have suddenly agreed to sell it for a few tens of millions then that's weird. As I said earlier, the lease was very obviously engineered to ensure Peel got the site back as soon as possible.
Just guessing obviously, but I can only think they'll write some sort of buy back agreement where the site can only be sold back to Peel at a very cheap price when the airport project collapses again, or else some environmental or geological problem has come to light that effectively renders the site worthless. Although it's hard to imagine that such a problem would only just have come to light.