business case is an intra-organizational business plan for an initiative or project inside the organization. It includes the purpose of the plan and its impact on the organization. It also provides financial measures to justify the feasibility of the initiative.

You can also simply say طرح تجاری or طرح توجیهی because from the context the reader can understand that it is inside the organization and it is different from a business plan for a totally new business.

It was already asked although I don't agree with the answer that was chosen by the asker:

http://www.proz.com/kudoz/english_to_farsi_persian/accountin...