SQL Multi User Procedures

Im using MS SQL and have multi user question...

I have a Customer Database with Customer Table and Payment Table.
I have two workstations - one for Customer Service and one for payment processing.

Here's a scenario -
1. John Smith calls Customer service to change his address and review Account Balance. Customer Service opens his account, displays his info, enters new street address but does not save changes to Customer Table immediately.

2. In meantime, payment clerk process payment from John Smith and also changes his Phone Number in customer Table, Account Balance and saves it.

3. Now, Customer Service rep saves changes made to Customer Table.

Now here's the issue....
How do we save only changes made (Address) without overwriting any other info for John Smith?

I know we can do Begin Transaction Where Cust='JSmith' and do our changes and COMMIT.

Is there a way in SQL to only update one column (address) for Smith rather than attempt to replace all columns for Smith and overwrite Phone Number and Customer_Account_Balance that was changed by Payment Clerk while Customer data was on screen on Customer Service Terminal?

Looking or suggestions....

Any help is appreciated