Let's look back at the triangle of cost, time, and quality. If you are constrained on resources (typically cost), you can control the time and quality. So you may work faster and with less quality. Or you may renegotiate and deliver some of the work in a second phase or project. Even better, look for an innovative solution that enables you to complete the work as planned.
The terms associated with this are "crashing" and "fast tracking". Crashing means you are going to add more resources. "Fast tracking" is seeking ways to do more work in parallel.