What I was surprised of when reading about this kind of topics is there were no alerts/limits on exceptional values. Like automated sales orders which were selling stocks for 0.01 cents when bought for tens of dollars. If extremes arise monitoring or even manual agreement will catch the biggest ones I expect. You can test those borders.
Off course every step should be logged traceable so repeating an error will be less likely.
Generally, if we are talking about risk, building financial related software on your own: You are the most obvious risk. There would be need for good audits to ensure you don't do strange things or make a mistake. For example misunderstanding a business case.
If you go for really sure it's interesting to take a look at the most exceptional ways of working, for example: http://www.fastcompany.com/28121/they-write-right-stuff so you could maybe pick up some pieces of it. Don't take it as a 1-on-1 example but there are some good examples to be learned from the "best" in the industry.
Edit: Based on your comments. The limits etc. can be defined in agreement with business. That way you shift the risks to business more (if you implement and test them well) and you also give a more clear view of risk to the business. That is a good point because in the end business is responsible. If the tests / risks are clear for them it is more clear how to handle and estimate them.
Based on the relatively small scale I have the feeling that communication and openness with the business might reduce most risks.
Concrete: You might have readable tests (integration likely) in which some kind of user stories are readable. Like:
As a junior business guy
I enter a trade order for Testcompany
With amount 100.000
Which is bigger than junior level limit (of 50.000)
Results in order with status pending
Results with e-mail to manager of junior business guy for acceptance
That way you can communicate about the business rules in a very clear way with the people handling the business. It's like a contact between development and business.
It's also discussable, because business understands. So, if they want a change you can together look on the relevant stories and see the consequences of for example raising the limit of the junior. Result is the transaction will proceed and senior will not be notified. Business can then define whether that is the correct step.