When your budget is fixed, and you’re unable to increase it, cost becomes your most important constraining factor. Your only option for staying within budget might be to reduce scope.
Cutting the scope usually reduces costs. You trim the scope by deleting tasks or by reducing the scope of individual tasks. Either way, you’ll decrease the total number of work hours in the project. Thus, you can pay fewer resources to do the work that remains or pay the same number of resources to work fewer hours, lowering both resource costs and overall costs.