Safe/correct way to reduce balance from account

Transact-SQL
1

i have a table:
id int,customerid int,balance.
i want to reduce for a "customerid" from his balance,assuming his balance-@amount to reduce>=0.
what is the corret/safe way to do it, assuming the reduce can be called more then once at the same time from different applications?

2

Each should be it's own record and then you can add them together

3

Create a rule for customer id for balance >=0;

The different application should commit immediately after each call, to avoid bulk reject.