Hi Maria,
You can do milestone billing without sales order.
1. Create planning profile and assign revenue element in"revenue element for billing plan" (Transaction OPSB). Do not check 'Automatic Revenue planning from sales order'
2. Configure Billing plan settings for WBS Element in SPRO. Also you can assign a default billing plan here.
3. Create billing plan for WBS Element from CJ20N. Select the WBS Element and click Ctrl+F5.
4. Once the milestone is achieved you can do milestone billing from FI ( not from sales order)
Please let me know if you have any questions.
Thanks
Regards
Srinivasan Desingh