I don't have a lot of payment experience, but AFAIK actual payment systems work in an append-only fashion, which makes concurrency management easier since you're just adding a new row with (timestamp, from, to, value, currency, status) or something similar. However, how can you efficiently check for overdrafts in this model? You'd have to periodically sum up transactions to find the sender's balance and compare it to a known threshold.

Is this how things are usually done in your business domain?