Let's break this down into two parts.
First is challenging the commonly received wisdom that banks use Oracle over Postgres and MySQL. Questioning things like this is a good idea but in this case the evidence is pretty straight forward as a quick Google will give you a fairly complete list of oracle banking case studies, Postgres doesn't. While I'd hesitate to say this is authorative, I'd be surprised if Postgres weren't shouting from the rooftops if the product was being used in this way.
Add to this the mass of anecdotal evidence that Oracle (and the other big boys) is the defacto standard for this sort of system and I'd suggest that you need to be finding solid evidence that it's not the case, rather than the other way around.
I wouldn't dispute that Postgres (or MySQL) is being used by banks, just that it's not being used for the sort of mission critical, high availability, high volume systems you talk about.
And that moves into the second part. What are the requirements here?
For an ATM system I'd suggest you do need high availability and absolutely top quality support (who do your developers call when they need help at 6pm on a Friday?). You need to ask yourself what platform can provide that.
If you believe that you can architect a Postgres / MySQL solution that meets those needs to the standards a bank would expect (remember, these are very conservative people as a rule) then go ahead with it but I suspect that they'll have questions about support and reliability and will want to see someone else using that platform for that sort of solution rather than being the first to take that sort of a risk.
I get the impression you want Postgres to be a contender and that's fine, but it's your reputation at stake here if you're making the recommendation so you need to be absolutely confident that it really can do the job and not let what you want to be true cloud your judgement.
Finally, I'm not saying any of this is right in terms of how things should be, just that it's the way things are from my experience of the banking and finance sectors.