Lonnie didn't wait around. He quickly took out a soul contract from his storage ring, wrote down the contract, and made sure that there were no loopholes that Kai could exploit.
Although he was taking a huge risk by letting Kai leave alive, he also had a secret plan that would ensure that Kai would die after he fled with the treasure.
Since he can't kill him, it doesn't mean no one else can. Once this place was revealed to the city management, they instantly found Kai. Since Kai could answer because of the soul contract, he would die by either soul search or by simply opening his mouth.
Even if someone finds out about him taking the treasure, he will be long gone, and he makes sure that Kai will protect this secret as long as he can to buy enough time for his escape!