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 a common thing now, especially on Bybit which is their den. I've experienced this bullshit up to 5 times on bybit which made me abandon it and seek Bitget which I believe more matured traders operate upon.
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.
Personally, before I even tell you of the Incomplete price, I would first appeal your payment and the scammer's second punishment is that he would beg tire before I would even consider releasing his coins. I believe a negative review would be very appropriate to warn off other people who would want to trade with the scammer and put a dent on his reputation