CC knows Law won't kill Monet because it would not help him. And if he did, CC is proboly willing to trade Monet for Law. If CC wasn't thinking rationally he would do what you are saying and just give Law his heart back. I think there's something more to Law's decision to give cc his heart, and I think it might have something to do with Law's crew. Law isn't stupid, and giving up his heart isn't just reckless, it's desperate. maybe the lives of his crew depends on him getting the mystery "key" on the island.
If he said "No, I won't agree to that" this 'key to throwing the New World into chaos' whatever it is might of been lost to him; especially considering that CC is a genius(supposedly) and that if this key is something of his arsenal (most likely Smiley in this case) he would have measures in place to make sure it would not fall into Law's hands if he simply went hostile against Caesar. Look at it as an investment. Law has a plan and knowledge of something that is probably in his opinion his 1 chance of progressing with his crew in the New World without being forcefully subjugated by a Yonkou. So in this case he invested a part of himself in this case his heart to guarantee that he could obtain this "key" he wants so badly. And if this "key" is capable of destroying a Yonkou then it certainly would be well worth it. Because face it, on his own his crew would be crushed by a Yonkou's without something special.
Let's talk about Poker for a second, Texas Holdem. This is the blind. You have Pocket Aces. Someone goes all in. Do you follow up by going all in, or do you fold? That's pretty much what Law did, he bet his life for a chance of winning big. And from what we can see, it is a pretty damn good chance, as good as he is going to get, considering all the effort Law put in to find CC in the first place, and willingly give up his heart. The reward potential alone is probably enough to justify the "gamble" of giving his heart to CC. And I don't think that he was without plan either and he may even of considered the possibility of CC asking this before meeting him. Monet and Vergo are technically Doflamingo's subordinates so if he underestimated anyone, it was DD and not CC.
Biceps, there are risks worth to be taken and risks that aren't. And is not about the reward only. More important is the chance of obtaining that reward. One could think that selling his soul to the devil could make him immortal. He would take a risk. But he would take a dumb risk cuz after giving his sould he would have nothing at his hand to take that reward. Not everyone that takes a risk is a winner. Most are losers. And one can be a winner exactly by not taking a risk that would lead to his lost. Luffy out sources his intelligence requirements to his crew. Law doesn't, so Law is as stupid as he apparently is. Why did he get MONET's heart. Wouldn't an exchange of both their hearts work better!? Unless CC has some sort of invisible suggestion gas that makes people more likely to follow along with what he says in a non-hostile or familiar situation; Law blundered.