The people really holding the cards are the Malkinsons. They could choose to sell the ground to the club at any time, but they want to get as much as they can for it, assuming (rightly) that the ground would be worth more redeveloped for housing or retail than as a football ground.
However, for it to be viable for a developer, the council would have to indicate that they would rezone the land to allow a change of use which they can refuse to do. When Sotnick put forward his grand plans to move the club, the council made it clear that they would not allow any redevelopment of York Street unless a suitable alternative ground was available for the club. I’d hazard a guess that building a new ground near Boston with associated development is not exactly an enticing commercial prospect in this current economic climate, especially when the club can't contribute the profits from selling York Street as part of a deal. So if the club made it clear that a new stadium is not going to be easy and there’s no guarantee it will even happen, and the council made it clear to the Malkinsons that they will not allow redevelopment until there is an alternative ground, then the Malkinsons may realise that it’s in their best interests to sell the ground to the club while the club is still a going concern (or at the very least agree a new lease with them). Otherwise in the worst case scenario they’ll have an asset which provides no income as the club has gone bust, they can’t redevelop the ground as the council won’t allow them to still, and there are hundreds of irate United fans happy to tell them to their faces how angry they are.
After all their years owning the club, you’d hope that the Malkinsons might still have sufficient feelings for it that they would consider doing a deal that would secure its future rather than try and hold out on the assumption they can sell to a developer while putting the existence of the club at risk as a result. Max Griggs even gave Nene Park away to the fans after having rebuilt it (although this still wasn’t sufficient to stop Rushden & Diamonds going bust).