d. - to get the best contract he can get.
when the Pats contact the Vikings they say, "if we agree to get Cassel to sign a contract with you, we'll then tag him and trade him to you for X." They then would have to go to Cassel and say, "Matt, we're going to tag you and trade you to the Vikings if you agree to sign a long term contract with them." What are the chances that Cassel's agent believes the Vikings contract offer is going to be anywhere near what he can get if he refuses and the Pats have to release him?
Two unique factors here:
1) Nobody believes under this scenario that the Pats actually intend on keeping and paying Cassel for $14 million for a year so if he refuses, he knows he's going to likely be released.
2) Whatever the Vikings offer Cassel in terms of a long term contract is going to be minus whatever they would have to give up to the Patriots in order to get him. Since Cassel receives no direct value from that, the Vikings would have to give up some value to the Patriots and then still give him a contract which isn't markedly lower than a realistic estimate for what they would have to get him if he went to FA.
So the Vikings are going to say to themselves, "what is the additional value that we are willing to give up in order to keep him from hitting FA?" If he hits FA, they fear they might have to pay him more so just in case, they are willing to give up the additional picks to the Pats AND give him a contract that's close to what Cassel's agent thinks he can get in FA? I really doubt it.
That is if they're even allowed to collude before hand. I'm not saying they aren't but with Tebucky, his franchise tag wasn't so monstrous that they couldn't afford to keep him if they had to so I don't know if they had the trade worked out before hand.