The incentive is to not ask for a transfer and get a cut of the fee, Given was trying for that when he left and we had to pay Barton £300,000 to get him to sign as Man City wouldn't as they claimed that he'd asked to leave. That £300,000 was what he earned on a £5,500,000 transfer so if Carroll was due the same he'd get over £2,000,000.
As for threats, such as? The biggest threat I can think of would be one coming from the player and directed at the club, again like Given.