Hello Supreet Bhatia,
I came across this unanswered post and wanted to check on the current status. Did you find a way to get the data source from Teradata to update as needed? If the connection is live, I would expect the values to update dynamically however this sounds like a Teradata extract and if that is the case, then the extract will have to be refreshed.
Please provide the version of Tableau and Teradata driver in use.
One other option I can think of is to utilize a view of the Excel data to act as a action filter to a view of the Teradata Data with the TeraData view empty until customer id from Excel side is viewed. This would eliminate the use of Custom SQL and likely work faster.
I hope this helps.