Track the Whistleblower

In the early 1990s, I received a handwritten IOU on company-headed letterhead, asking for a loan of 2,000 yuan. The reason was to buy a house. The borrower, Zhang Hu, left me the written note and mortgaged a case. He knew I was interested in the case and kept asking about it. If you can't pay it back, you can take it apart and look at it, which is equivalent to using the case as a guarantee. He never paid back the money, and I never pressed him. Until one day, I accidentally learned that Zhang Hu had died of a heart attack while traveling. I opened the case in the secret letter and began to crack the case.