Successfully managing client relationships, especially in these pressured times, takes the combined efforts of skilled interactive agency professionals. More PM's have lost more hair (or at least their coloring to gray) hearing phrases like, "XYZ Account person told the client that, we could do "it" and still meet the schedule. S/He made no mention of a potential fee increases."
I have toiled, presented, implored, cajoled, collaborated and otherwise tried to influence agency Account people not to play the game of, "Client says, 'jump', we say, 'how high'?" or to look at the advice/insult another way, to stop being "yes-(wo)men" (I can assure any offended Account people that this is a mild characterization compared to those bandied about an agency - especially in the wee hours of the morning.) Among the many benefits of removing the instant, "yes" are that it would take some pressure off of the PM's, who invariable are vilified when they assess the situation and have to say, "no".
The aim is to migrate towards more of aconsultative answer of, "maybe". Ideally, towards an Account Management-response along the lines of, "Client, I understand the request, let me talk to superwoman-PM back at the office, see how we can address this. I will get back to you ASAP with a recommendation and let you know if there are any impacts to the project's scope, timing or pricing." This is not a bait and switch. It actually gives the agency some time to thoughtfully address the request and be fairly compensated if there is a legitimate change in scope.
When discussing this issue with a wise agency president, he suggested that one way to facilitate the migration of yes-(wo)men on the Account team to the maybe column was to also move the PM's to the maybe column. He posited that one reason PM's are often met with resistance or, worse avoidance, is that in their zeal to defend, PM's can become very problem-oriented vs being solution-oriented - effectively turning themselves into "no-(wo)men". If PM's reflexive response was also, "maybe, let's see what we can come up with", the partnership would benefit, friction diminished, and the ecosystem would be better aligned for success.