Earth peak disciple.
Of course Ling Xiao can't go and steal TianXin sect's alchemical technique for training the soul force. That volume of alchemical technique is kept on the highest level of the East pavilion. Guarded by Elders of TianXin sect that have retired to work behind the scenes. Each of them eccentrics with great strength. But this doesn't mean that he can't get that volume of alchemical technique, it's just that if the alchemical technique is stolen, there will definitely be a huge uproar in TianXin sect.
That volume of alchemical technique is deemed as a very important treasure for TianXin sect, so it is checked on frequently. Once the theft is discovered, TianXin sect will definitely suspect that there is a traitor within its ranks. Then that group of old guys will definitely appear from behind the scenes to examine each and every disciple. This would be very troublesome for him. Furthermore, You XiaoMo himself seems to have quite a bit of secrets. So stealing the alchemical technique is a very bad move. He won't choose this method except as a last resort.
But he already told You XiaoMo that he would get him a volume of alchemical techniques to train the soul force. As a trustworthy man, he must do what he says he would. Hence the next day, You XiaoMo learns from others that Ling Xiao has gone down the mountain.
Yesterday, after finishing their discussion, Ling Xiao stayed for a while before leaving with twenty magic pills. This leaves You XiaoMo with thirty magic pills. He feels that he can rely on selling these for money the next time he goes down the mountain. Even though it is not a lot, it would at least stop his money bag from becoming flat.
As for why Ling Xiao went down the mountain, You XiaoMo unconsciously thinks that he could be going to look for alchemical techniques to train the soul force. He feels a little guilty in his heart, since Ling Xiao is helping him look for it even though it obviously has nothing to do with him. But when he considers that after obtaining the alchemical technique he will be able to refine even more magic pills, that guilty feeling slowly disappears.