What should be done if someone accidentally sends an excess amount of

I made a credit card Paytm on 2 nd March but that transaction got cancelled and i received refund after some time I got credit card payment by Paytm and also refund to my account now Paytm is threatening me to pay the excess amount i recieved should I worry about this because I have there money should i reply can they take any legal action against it please say and also they blocked my paytm account which I have kept 2800