There are no official Cheats for Hay Day. The game is regularly patched to combat any glitches or hacks, and anyone found actively trying to subvert the Diamond system in the game or gain access is likely to lose their account and risk not being able to get back on. If you get another player offering you free Diamonds in exchange for your details, it is going to be a scam. They will be after your Facebook account, most likely.

The developers of Hay Day have worked hard to create a game that is free, but still enables them to earn money by making some tasks just tempting enough for people to spend Diamonds on. If people were able to get these things for free then the developers would lose interest in making further updates and keeping the game servers running. So keep playing and, if you don't want to spend any money on Hay Day, you don't have to!