Agree with the above response Michael's response! I am sharing some additionals:
1. App documentation - If you don't already have it, write up a very detailed requirements doc that explains purpose of the document for the users, has wireframes with functionality/purpose of each screen described including error (& other) messages to be displayed.
It may sound like a lot of upfront work but it's worth the investment in time to think through as the offshore folks will mostly just execute on your requirements.
It's the little things that will accumulate to be problems.
2. TESTING - Make sure to check how they do testing. Which versions of devices & OS will they use? They will need to send you an issue log to agree on any outstanding ones before each milestone payment.
You will need to contractually & verbally agree that you will need to do user testing after certain milestones/builds are considered done from their perspective. Testing can be the most challenging part of iOS apps if code is not fixed with version control/discipline.
3. Make sure you have a contract with them which states -
(a) IP developed is yours. Technically there is nothing you can do if they choose to reuse it. But you can at least have this in writing for another US client.
(b) Handover items - should include (i) all xCode used for the final app product, backend database files if any and some documentation. (They might balk at detailed documentation or charge you extra. But there should be a very minimal level to help you understand what's what). After the final payment is made, handover should be completed.
(c) Deployment assistance? If you need any of their help installing the code, db on a live server for you. And testing to make sure it all works
(d) Post deployment support - what will be the level of support per day/week & what items will they support.