Myntra.com — the product picked up from me, but no refund or product back

The product i put it for exchange, mistakenly gave back to delivery agent with other return product. When other delivery agent came for exchange the product, he is asking for product to be exchanged. But i already gave back the product with other delivery agent. When i called myntra regarding this, they are telling they didn't receive the product back. They are declining the refund or the product back. They are asking me for proof, i don't have any proof only my friends are the witness. When i called the delivery agent he said he exported the products back and he will give the receipt. But now he blocked my number. Please do help me with this.
And also in myntra app it is showing return request declined, but i didn't kept it for return. I kept it for exchange.