According to Apple guidelines:
What url should I use to verify my receipt (iOS)?
Use the sandbox URL https://sandbox.itunes.apple.com/verifyReceipt while testing your application in the sandbox and while your application is in review.
Use the production URL http://buy.itunes.apple.com/verifyReceipt once your application is live in the App Store.
How should I do that? Do I send an app with the sandbox URL, and once reviewed I have to compile once again with the production one? I guess this would violate some review guideline, or would make reject the app when they realize the CRC or something isn’t the same as what they tested, wouldn’t it? Should I make some page in the middle???
I have the hope once validated, Apple would modify the bytecode in order to set the correct URL heh.
Where are you storing that URL?
According to the suggested setup from Apple, your server validates your receipt with iTunes after it is called from within your app. Your server then sends a receipt validation response back to your app, and optionally an upload URL for any uploads.
So the URL is located on your server, in a PHP script or similar. Once your app is approved you would change your server script to point to the iTunes production URL.
Therefore there is no need to update your app code after the review. In fact if you do, that requires the submission of a new version and a new review.