Automatic query to fill column

I'm using a software that calculates prices for aluminium structures.
When the project is finished, the project data are stored in "PROJ" table. This table has 2 columns for text comments, "UserText1" and "UserText2", taht we usually never use.
We also have a "CUSTOMER" table, where we have a column "Price.tariff", where we is assigned the name of the tariff for each customer.
A column with this same name - "Price.tariff" - exists in the table "PROJ". usually the values in the 2 tables are the same. 

Because sometimes this  "Price.tariff" values aren't the same in both tables, I wonder if is possible to use one of the text columns of the "PROJ" table to store an automaticly generated text, every time this values don't mach.
Once the project is saved, an automatic query would run, and after comparing the "Price.tariff" values in both tables, an automatic generated text would be stored in one of the text user columns.
Questions:
How to create an automatic query that runs when the project is saved, and what could be a simple way to write it?
Thank you ins advance for any help.
Jtpg