Never allow it. I just severed ties with a client today because I DO NOT like to do fixed cost projects in consulting. There are always changes or sometimes complete redos. I also had a 3rd party who was not on Contract starting to give input that was the opposite of of my actual clients. This party is NOT on the legal agreement.
Or if you estimate hours and you want to earn the project be realistic. Do not underestimate or compromise your billable hour to get the work.
You are setting the tone of the whole project and potential long term relationship.
Be loyal, be assertive, work hard… but never ever compromise your self worth for the sake of a short term project.