this would be a clear case of "Restrictive Trade Practice" or as they say in the US "Anti Trust practice"...the institute is misusing its dominant position of providing you the admission before other major B-schools did. I am sure SIIB (and many others instis) did this on purpose - they are fully aware of what they are doing, but since legal recourse is quite messy they may venture to do so.
you can/should prove that you were *forced* to sign on the dotted line, since there were no admission offers at that time and cite that the deadline allowed for refund is much before results of other B-school are announced.
Also, why should an insti charge 10% (or whatever the amt) as retaining fees. All the costs of admission should have been recovered from the app fees we give at the time of applying. Since they are recovering from the next student in WL, I feel this cost is not justified.
Since there may not be clear laws in India abt this situation, its best to check what laws other countries follow - For IP related laws few cos. refer to US since India doesnt have such strict laws. May be you can take a cue frm that. For this you first need to get hold of a good lawyer. personally I dont have experience with Consumer forums, so, I wont comment on their efficacy.
This will certainly take time, but be sure you give some publicity to it

so, the next time the instis will be careful!!
Just make sure all correspondences are in writing since written documents carry more weightage in legal tangles. Send a request for a refund even though you have signed that agreement. DONT send emails to insti, send Fax, Regd Letters with Acknowledgements - in these cases sticking to the old forms of communications is the best option.
Best of luck and do keep us posted on the progress....
cheers
subhash