Hello Rahul,
According to the official website of CSAB they will provide the refund at the time of registration for Special vacant seats filling round.

• The board has announced that the refund would be made from 23.08.2019 if the candidates fulfill the criteria that are listed below :Candidate who got a seat allotted, did not report in NITs / IIITs / GFTIs either during RC reporting or Institute reporting (19  23 July, 2019) and also did not participate in special vacant seat filling rounds by CSAB.
• Candidates seat was allotted in JoSAA rounds but was cancelled subsequently due to the ineligibility condition and did not participate in CSAB special vacant seat filling rounds.
• Candidate opted to WITHDRAW the seat allotted in NITs / IIITs / GFTIs during Institute reporting (19 - 23 July, 2019) and did not participate in special vacant seat filling Rounds by CSAB.
  

The below given link might help you :
https://csab.nic.in/cms/Public/Home.aspx

• Good luck!