Projects often spend extra to make a deadline
Alan gave a good answer pointing out the limitations of the project management "iron" triangle.
To answer your specific question, you need to consider that projects (not necessarily software projects) often plan to spend extra money to make a tight deadline. Examples of such extra spending to make a deadline are:
- Overtime: You may be planning to pay double time (for extra hours during week days) or triple time (weekend hours).
- Expedite fee: You may be planning to pay expedited fees to vendors to do rush jobs.
- Air freight: You may be planning to pay for air freight to get supplies from vendors earlier than if you shipped by surface freight.
If you have contract resources to whom you need to pay only for the hours actually worked, it may be possible to save money by extending the deadline. So, to answer your question:
Well, if I extend the deadline, I will need to pay for the resources
working in this extended timeline, so how can I "only" extend the
As seen in the example above, if you had more time, you can avoid paying double and triple time and get increased scope of work done at same cost.