If I was the Mc I would have played it off as if the bank can be considered as a market. For example, li just had to tell the elder that someone put up 5 years for trade and he wanted to take revenge in order to use the trading platform something so mystical that overrules the laws of life and death then something precious has to be traded.
The MC used his life span of 10 years (lies) but the bank took 5 as compensation. While the elder li bought those 5 years and the bank took the spirit source as compensation.
This way li would be seen as savior in the eyes of elder li and given more benefit. While he can also tell the elder that he became an employee as well. Giving him more weight.