Hello there!
I think I understood what you need to do, but as far as I know, that functionality is not available to the custom scripts. There is no way to detect a "click" action by the user, let along hover, directly in the Google sheets interface.
This kind of events are easily detected on the usual webpage through some JavaScript, but Google doesn't let the developers use it.
Just to make sure, I understood you, you need to fetch the data from another sheet once user points to a particular item on the Expense list. If that is the case, you might need to think of some other way to implement it.
If I misunderstood something, let's have a chat, and we will have it sorted.
Cheers,
Denis