FWIW, we're not entirely sure if you're asking specifically about data modeling (like a general CS question) or a question specific to Rails/ActiveRecord. You probably would do yourself well to get advice about your system from someone with computer science experience, specifically building transactional systems.
Generally speaking from my own experience with transactional systems, ActiveRecord has some drawbacks that don't optimize for scale. In particular, if the set of operations requires lots and lots of objects to be loaded during any given transaction, ActiveRecord has a high cost (slow) associated with object instantiation. This doesn't affect most Rails apps if they create 25-50 objects in the lifecycle of any given request, but when a request is going to create hundreds and hundreds of objects you're going to see some bottlenecks at scale. (I'm not sure if this really applies to you -- just some background as you are going down this path.)
This doesn't mean that Rails is the wrong solution -- it just means you have to think about the bigger picture and perhaps know a little more about architecture than you do right now. Other alternatives to ActiveRecord (Datamapper) do not have as high a cost of object instantiation so might be good options for a problem like this.
There are yet other advanced design patterns for transactions and banking systems too -- for example, just check out all the discussion about things other people have solved before you on the "Transactional processing system" Wikipedia page:
http://en.wikipedia.org/wiki/Transaction_processing_system
All of those concerns are out of the scope of Rails & ActiveRecord.
If I were you I'd learn a little more about your domain problem and back up and research different options, then prototype some in Rails and see if you measure how fast common operations take.
-Jason