Apparently there are tools that enable retailers to upload offline transaction information to Facebook
https://leadsbridge.com/blog/facebook-offline-conversions/
"What is Facebook Offline Conversions?
Facebook Offline Conversions enable you to upload your sales data to Facebook and match the transactions that take place offline with people who have seen or clicked on your Facebook ads. If there is a match, Facebook attributes the sale to your Facebook ads.
According to Facebook, “With offline conversion measurement on Facebook, you can track when transactions occur in your physical retail store and other offline channels (ex: orders made over the phone) after people see or engage with your Facebook ad.”
There are many reasons why Facebook Offline Conversions can benefit businesses.
For example, a car dealership business can set up Facebook Offline Conversions to know the number of people who bought or serviced their cars as a result of seeing or clicking on a Facebook ad."