Hi MBBS, I'm Adv. Niharika and here is my take on your query.
First of all, if these demands and their transactions thereof were done personally by some college official and in cash (by which i mean, in black/ under the table) as a common trend in your college/university. Then there isn't anything that you can do about it now as you agreed to pay that amount in order to appear for re-examination under an illegal setting and without any record.
Secondly, if the college had laid down any rules/guidelines with respect to the re-examination fee and process, then such an irrational amount of fee can be challenged in different forums and even in writ if need be, however the success of the same cannot be guaranteed as such. Go step wise with levels of escalations for the complaint :
1.Vice Chancellor
2.University Grants Commission
3. Secretary Ministry of Education and HRD.
You must also keep a copy of letters, and replies (if you receive any).
However, in my opinion, there are only bleak chances that you will get a refund of the said amount.
Hope this information helps. For further query, you can reach me at niharikalohan@yahoo.com