Hi Husain,
i I am not an official representative of SAP on this topic, but I can help you with some inputs.
the methodology is accessible at service.sap.com/ASAP
you can find plenty of materials by Jan Musil in the landing page of ASAP Methodology for Implementation.
regarding Business case. It is a PMP concept that a Business case justifies the Cost vs Benefits analysis or a market requirement to create a project.
typically techno, legal, environmental, competitor, social, innovation relayed triggers could lead to the business case.
so without the business case you cannot justify who need to sponsor the project. If the impact or benefit is to the finance department then the CFO could be the sponsor, if the impact is to the overall vision objectives of a project then it could be the CEO etc. to just give an idea.
A project sponsor will need this business case to approve the project thereafter which a Contract or SOW will be made before creating project charter objectives and deifie the success criteria for the project.
Thus the Next document after the Business case will be a SOW/Contract leading to a Project charter that will authorize a project manager to start the project work.
so now business case is PMP terminology and ASAP is 100% compliant to PMP methodology and hence it will be great to have a business case. In SAP world the business case could also be called as a value case and hence found under the area of "value management"
hope this helps you for a start, please let know if the answer is helpful.