Hi Tawni,
This can be mapped through standard subcontracting process. You don't have to use sales BOM. Production BOM will be workable solution here.
In this case you are the subcontractor. You will get the free material through subcontracting stock transfer. After producing the material you will return back to the customer. Intern you will get the conversion charges.
You have to select special procurement key in material master MRP2 view as subcontracting (30).As well you have to set the control key and allow external operation.
Kumar has posted two documents regarding subcontracting process basics for both operation and assembly subcontracting.Please go through it. This will be helpful for understanding.
Subcontracting process in Production
Assembly Subcontracting in Production
Hope this helps.
Regards,
Krishnendu.