It can be done. I never worked on a project that starts with the manual, but I heard about such projects.

There is really no problem in writing the manual before any coding is done. It just needs a very good understanding of the problems and a good knowledge of the tools involved (so you don't write that the program does something that cannot be done :)).

Obviously, you must be payed for this work. When the manual is done, the functional design is done, so, an important part of the project.

You can revers thinking the project: You get the manual of some software, but you don't get the software. If the manual is detailed enough, you can rebuild the software based on the manual, right? So, why not start by writing the manual?

Vlad