No broker will automatically protect you from overpaying, it's your responsibility in any case. Before signing anything, spend a week researching real apartment prices in the area you're interested in. There's enough publicly available transaction data in Dubai to get a rough idea yourself. You can check sales information on the DLD website for free. For example, you can view current listings for apartments for sale here
https://atvir.com/uae/residential-complexes/apartments This way, you can get a realistic price range before negotiating with anyone.