You can use Normal Sales Order (VA01) and Purchase Order-regular process without using document type UB.
1- You need one of receiving plant as Customer so that you can create SO
2. You need to create issuing plant as Vendor you can create PO
3. You can use any price as per your requirement in SO without going through much developments and post the billing document to right accounts as well
Or I am thinking you can try FI Substitution rules to influence your accounting document-
How to define a substitution in FI with an example. - ERP Financials - SCN Wiki
These seems to be standard options to me.
PO and SO you can automate too using EDI settings as well.