"The phrase '天上掉馅饼' in Chinese can be translated into English as 'a pie from heaven' or more commonly, 'a free lunch' or 'something for nothing.' These expressions convey the idea that something good is unexpectedly coming without any effort or cost, which is often viewed with suspicion or considered too good to be true."

If you're looking for an exact idiom match, "a free lunch" or "something for nothing" would be the closest equivalents.