You work on an important project that contains 7 independent modules, and you do not have enough time to finish the project in time. In front of you is a choice: to complete 3 modules in full or to start work on all the 7 modules in parallel, but not complete any of them within the planned period. Which strategy you choose?
Explain the situation to the manager, let him decide. That's why he's there.
If you're in charge, then you need to consider all possibilities. I'd say, focus on what the customer will need in the first turn, and complete the second-priority functionality afterward. If possible, talk to them, explain and try to reach an agreement. The customer may have a different opinion on what he will need the most and give you direction to focus your efforts in.