I think the idiom you are sseking is "It is too good to be true."
This idiom can be used positively or negatively.
A person receives a surprise award from his colleagues. He says "This is too good to be true." It means that he is very happy and never expected anything so wonderful to happen.
A person receives an e-mail. You have won a contest. You're prize is an all-expense paid vacation in Florida. Just call this number (charge is $2.99/minute) to collect your prize. The person deletes the e-mail, saying "That sounds too good to be true." In that case, the person believes that it is a false prize and it is going to cost him money.