Actually, I do not need and additional field, I simply want to use the uos from the product. I guess I can patch the hr_timesheet_invoice module, no?

I fixed by patching hr_timesheet_invoice. I believe this is bug in OpenERP. If I set a uos then I want my invoice to be in uos and not in uom. That's the whole point of different uos and uom, no?

Here is the original code:

                ctx =  context.copy()
                ctx.update({'uom':uom})

Insert 3 lines in between the 2 above lines:

                uom = product.uos_id.id or product.uom_id.id
                if product.uos_id:
                    qty *= product.uos_coeff