gold does not simply vanish in CoE. if a contract is broken and you want to ensure the punishment, then you have to act. like going to the sherriff or headhunters, find the person, take the gold from them. if the person breaking the contract hides in the woods and is never found again, there is no punishment.

the thing with the contract getting fulfilled is a bit harder. i guess, contracts will be simple things between players. you get a contract to deliver a letter. you deliver a letter, give the guy you gave the letter the contract, he pays you. or he gives you something to show you have delivered and you go back and with the contract in hands, get your money. if no one knows that the letter was changed inbetween, nothing happens. but thats just my guess. i dont think, that you can put the Item ID or something in the contract. but it shouldnt be easy to exchange a sealed letter. and i would put my seal on relevant letters, if i were a noble. like press my seal ring on it. as long as no one else has my ring or a copy of it, i should be safe. obviously, if i found someone that makes copies of noble rings, i'd punish him, if i were a noble :P

While there are certain mechanics that could make falsifying a contract or letter a lot harder, there will be a great deal of trust involved aside from the active enforcement.

But on the other hand, what is worth more? A single reward for that letter or a long career with the royal couriers? Unless you are very impulsive, you are probably not going to exile yourself from civilisation unless it's about something extremely valuable.

That is where reputation comes into play. I would not be handing a letter to anyone without knowing them a bit before or using a npc or pc family member to run letters for me. If you are sending a letter to family it was stated there would be a confidential way to do that in the game mechanics.