Hi Sunil,
There is no need to build a custom interface between Magento and SAP (I'm assuming you are on an R3 version...ECC or Hana). My company Corevist (www.corevist.com ) has had a cloud based version since 2011. Please go to our website to learn more about it and then we'd be happy to discuss. Just drop us a demo request and a consultant would be more than happy to speak with you.
I'm 100% certain we can help.
Best regards,
Sam