Is it OK to keep a value which updates in a table?
Yes, that's non-normalized, but occasionally non-normalized designs win out for performance reasons.

However, I would probably approach it a little differently, for safety reasons. (Disclaimer: I don't currently, nor have I ever worked in the financial sector. I'm just throwing this out there.)

Have a table for posted balances on the cards. This would have a row inserted for each account, indicating the posted balance at the close of each period (day, week, month, or whatever is appropriate). Index this table by account number and date.

Use another table for holding pending transactions, which are inserted on the fly. At the close of each period, run a routine that adds the unposted transactions to the last closing balance of the account to calculate the new balance. Either mark the pending transactions as posted, or look at the dates to determine what's still pending.

This way, you have a means of calculating a card balance on demand, without having to sum up all the account history, and by putting the balance recalculation in a dedicated posting routine, you can ensure that the transaction safety of this recalculation is limited to a single place (and also limit security on the balance table so only the posting routine can write to it).

Then just keep as much historic data as necessitated by auditing, customer service, and performance requirements.