@WangXian it's not explained in the novel as well... But what I could understand that whenever Land Zhan and Wei Ying parted their ways... Wei Ying always thanked him... So Lan zhan really hates it when Wei Ying says thank you to him... He fears Wei Ying would run away again.. That's why he took the pouch back.. And the thing that's explained in the earlier comment also adds up to it. Basically Lan Zhan was making sure that Wei Ying doesn't leave.