She suspiciously paid for everything in cash for years, then people realized why
Bye Venice
During their time in Italy, he found it strange that Anna only paid in cash. This was a little odd, but it was not a huge enough deal to him. The trip, of course, had to come to an end eventually. However, Anna seemed to have forgotten to pay him back as she promised.
However, Michael was used to this kind of thing already. He knew that the super-rich had a tendency to forget minor details such as this one. Michael eventually forgot all about it as well.