I guess you need to access the button control of master page. You should create a Button object and then instantiate it with the master's page button control. Place the code in Content page.
Button masterSaveBtn = new Button();
masterSaveBtn = (Button)this.Page.Master.FindControl("Save");
masterSaveBtn.Enabled = false;
FindControl() method takes Control's programmatic ID as a parameter. Here in this case, I'm assuming your Save button's ID on Master page is "Save".
Hope that helps.