Safe/correct way to reduce balance from account

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?

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

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

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