Well, we know Jesus did not ever deceive, so, he was telling him the truth, in fact, he told many people that their righteousness had to exceed even the holiest people they knew (the Pharisees). The man he was speaking to was so full of pride that he thought he deserved eternal life, he would not even look at how sinful he really was. His money was his weak spot it seems. Jesus was showing him and all the others that the perfection of God was impossible for anyone to achieve and, by that, he was pointing them to the only hope they had, Jesus Christ the Savior, the Messiah.
Then how come the OT God did (by sending or being a "deceiving spirit")?
The man he was speaking to was so full of pride that he thought he deserved eternal life, he would not even look at how sinful he really was
The man earnestly believed he did everything right. Having an honest but erroneous belief is not sin in and of itself.
His money was his weak spot it seems. Jesus was showing him and all the others that the perfection of God was impossible for anyone to achieve and, by that, he was pointing them to the only hope they had, Jesus Christ the Savior, the Messiah
No he didn't. He told him to sell everyhting and follow him. How many people do you know who give up all their vices and follow Jesus (except perhaps some monks)?