This is common on an exchange which I am not interested to mentioned on this forum but you can see it on the thread that I will include its link here. There have been several complaints on the this about this P2P scam:

This is an example of the scam:
Alert: Latest P2P Scam

Someone told me it happened to him 4 times out of 5 times he exchanged bitcoin to naira this month alone.

You may want to exchange ₦68900, the scammer will send ₦60900 thinking you will not know. That is how the scam is. If you send the scammer message, he will do as if he did not know that it is a mistake and send the remaining amount.

This is becoming common on the exchange. Is it because people are not reporting it or the exchange negligence. I will advice people to start to report the scam and also give the scammers negative review.

You can give anonymous review on P2P exchanges. Also you can wait for some days before given the scammer a negative review if you want to avoid negative review from the scammer. I hope this may change their intentions later if people start giving them negative reviews.

Advice:
Make sure you check the amount sent to your account before releasing the coin. But not only that, make sure it is the right amount because the scammers are sending an amount similar to the amount that you are expecting to receive.