some apps like sling tv come in both separate iPhone and ipad versions. go to the app store and click updates. click on purchases. click on not on this iPhone ( or not on this ipad ) and everything you ever bought should be there for free.
maybe asking for money again is because you bought this on a different itunes account?
the fact that device is synced to different computers shouldn't affect apps that you purchased and want to redownload from the app store.
is it possible that one iPhone is on your itunes account for purchases, and the other device uses your iCloud account for purchases?
iOS 9 brought us app thinning. your apps are no longer copied from your device and into the library. they must be downloaded from the internet using itunes and then synced to the device. . this should not affect you and is not a reason why you are asking to buy it again. regardless if its itunes or not, if something is purchased, then you should be able to download it on the iPhone / ipad directly by going to that previous purchase screen