Zest Money - refund fully not credited

We had purchased a product from Titan using Zest money and unfortunately we dint like the product and we placed a return. Zest money *st emi was due on *th Jan **** and we had initiated the return of the product and notified zest money to cancel the EMi loan on *st Jan **** via email. And the same has been acknowledged by zest money and asked us to send the vendor details and vendor acknowledgement where in it shows the vendor has accepted the return and initiated refund.
Unfortunately the refund from vendor to zest money took more time and in the mean time we were getting constant email and message alerts to pay our first emi on *th Jan ****. We had not payed thinking anyhow there will be refund from the vendor. The same was communicated to us from one of their executive stating not to pay the emi. But fearing our cibil score might impact we went ahead and paid it on *th Jan ****, thinking we might get refund of the EMi we paid. But these people just refunded the actual amount and denied paying back the interest amount. Stating you have paid it late and liable for interest due as per our loan agreement. Why we have to pay when we have notified them with supporting docs well before the *st emi due date.
Now they are denying to payback our full amount. So we need your help I. getting back the full amount.