Va'eira

Why does Moshe keep removing the Makkos right when Paroh says that he'll free the Jews? Paroh changed his mind every time! Wouldn't the smartest thing be to keep the Makka there until the Jews are well out of Egypt? I mean, at least by the third time, they should realize that Paroh lied the last few times, he's probably going to change his mind again this time too! The Jews don't lose anything by keeping the Makka there until they're safe, out of Egypt!