This module solved the problem for me.

https://www.odoo.com/apps/modules/8.0/invoice_sale_order_line_group/

Hi Raffaele,

I'm using OpenERP V7 and I can invoice multiple sales orders on a single invoice, just by select on tree view several sales orders and then, on "More" drop down button choose  "Make Invoices". A pop-up wizard appears and selecting "Group the invoices" does the job. For me there's only an undesirable result on this process: order lines for the same product in different sales orders won't merge on invoice.

Hi Raffaele,

W.r.t your query, I have gone through it and tried to replicate it. Few recommendations which I would like to suggest you which might cater to your needs are as follows:-

1) To have multiple sale orders and then issues several invoices, each one takes lines from multiple sale orders is possible in Odoo.

2) For this we need to follow few steps as follows :-

I) Firstly we need to configure in Sales Module our requirement of generating several invoices based on Sales Order lines. This can be done by following Setting>Configuration>Sales>Invoicing process> Generate invoices based on the sales order lines

II) Now once doing so we need to go to Settings>Users and here as per our requirement which User we would be used to create invoices we need to configure by following Settings>Users>Users>Technical Settings> Enable Invoicing Sales order lines

III) On doing so after confirming a Sales Quotation, when we create an Invoice we would get the option to create Invoices based on Some/Selected Sales Order Lines where we can select Line Item for which Invoice needs to be created.

In this way your query can be resolved and your requirements can be met.

Hope it helps

 Cheers!!

1. Via Settings --> Configuration --> Sales, make sure "Generate invoices based on the sales order lines" is checked:

2. Make sure all users you want to be able to do this have "Enable Invoicing Sales order lines" checked on their User record

3. When creating an Invoice for a specific Sales Order, select "Some order lines" from the options.

Optional: 

Install https://apps.openerp.com/apps/modules/8.0/sale_order_line_quantity/ - which improves this process by showing you the total items per line (if any) that have already been delivered and invoiced.