Let's ignore the source of this loan for now; we just need to find out who borrowed the money from Li Yiyuan. Without a doubt, the person who borrowed the money is most likely the one who killed him to cover it up.
Li Yiyuan was in a car accident after playing cards with Li Weimin that day. The killer's motive was definitely to run him over, retrieve the IOU, and completely erase this huge loan from the world.
If we follow the killer's logic, to make a huge loan disappear without a trace, several preconditions must be met.
First, the IOU must have been freshly written; the action must be swift, giving the other party no time to hide it. Second, no third party besides the borrower and lender must know about the IOU. Finally, such a large account transfer must not attract police attention
