Ask HN: Help with mobile app that works offline and sync data

At our startup, our target market is increasingly conducting their business, on-the-go. Therefore, we have to augment our browser-based desktop app with a mobile app.

Therefore, I'm now evaluating what it would take to develop a mobile app that could work completely offline and sync transaction data periodically with the central server.

I've read briefly about Firebase, Parse, Usergrid, BaasBox, master-master replication, and the CAP theorem. As expected, I'm royally confused now!

‎We already have an existing data storage infra, which is using Postgres. And since we're dealing with financial transactions, I'd like to stick with an ACID store and not some new-fangled JSON hotness. Our app also has HTTP/JSON APIs for various read & write operations. I can even consider exposing very fine grained RPC calls over Thrift/Protocol Buffers, which frankly, seem more manageable than the JSON APIs.

Now, my question is, is there a project/service/SDK out there which has already solved this data sync problem? What about conflict resolution when it comes to transaction data? I understand that this is like resolving merge conflicts in git, which basically requires human intervention, but is there an SDK that will reduce dev time for such scenarios? Can said SDK work with a custom backend API?

Also, how should one be storing all this data on the mobile app? SQLite?

This is the first mobile app that we're about to write as a team, and any gyaan from the experienced folks would be appreciated.