Bajaj Finserv — Unauthorized fund deduction

Please note had purchased a mobile in Nov 2019 through 6 month Emi option given by Bajaj Finserv and my last emi was deducted from my bank account in May 2020.
After completion of my emi in May they have deducted 117 from my account in Nov that too without my authorization.How can anyone deduct money from the account without proper authorisation.I had been purchasing house hold appliances using Bajaj since years and this practise has really broken my trust in Bajaj.I would want my money to be refunded asap as this is clear case of cheating.