HI pramodh555,
First of all - good question !
This is a an age old problem for BAs - do you do what's right or do you do what your told?
As a Manager of Projects and the Project Teams, I am responsible for both the BAs and the Project Managers, so I can probably see this from both sides of the fence.
First of all, a question. You say that there is already a high level Business Requirements Document? How long ago was it written and do you know the person who wrote it? Do they still work for the company?
If it Is fairly recent (in the last year lets say) then I would use that as your bible. It should have much of what your PM wants. I suspect that the PM may be looing to take that BRD and take up another level to get things to a state whereby they can actually be built. In which case, his request seems a sensible step.
However, if he is looking for things that go against that original BRD, then beware. If you can, speak to the person who wrote the BRD. Ask them what challenges they faced and why the project seemed to stall (assuming it did).
If the PM is trying to move things on, which is perfectly sensible, then make them aware of the BRD and ask them if they (the PM) believes any of the stakeholders have changed. There are two reasons for asking this. First, the PM may not realise that there is a BRD, so you are subtlety reminding them, and second, you will need to speak to the stakeholders to get the extra information that you require.
As a side issue, I wouldn't have thought Excel was the best choice of software to write what he has requested. It doesn't lend itself very well to lots of text. I'd do this in Word personally. A small point I know, but it will save you trouble later on.
To answer your question directly "Should I do what the PM want or produce a new BRD?" I would do both! I think the PM is asking for the next level of detail for the project, so I would take the original BRD, save a copy of it, and working on that copy (so you still have an original version available) add the greater detail as you go. This way you do what the PM wants AND you produce some meaningful documentation.