Is there a way to link or reference to a specific line in a transaction?

In what way? How would this interaction happen?

I want to create a relationship from a custom record that I have created to specific transaction lines, for instance transaction line on a Product Return.

If you don’t want to spin your own using the transaction no & line no, look at

id

on the line level - it’s a concatenation of the two. I can’t remember off the top of my head if it’s exposed in searches etc, but it’s there in the field explorer