Licencing options for developer libraries

This is my first ever post so be gentle with me

I have a client who is developing a library for iPhone apps which gives the developers some tools to improve the user interface (they won't want me going into much more detail). They would like to license the library in such a way that they can charge the developer a nominal fee per ultimate download of any app which uses the library ($0.001 for example).

Does anyone know what kind of licence would be appropriate for this kind of commercial agreement and whether Apple would allow such an agreement (or are they they only organisation who are allowed to make money on a per-download basis?).

Thanks in advance, Alec

If the libraries are being released as binaries, then the licensing is essentially proprietary and your client will sell them on their site charging whatever they want and developers would use the library with the accompanying API to talk to it.

When the application is submitted to the Apple, the library would go along for the ride and as long the library follows Apple's restrictions just like the application would, there shouldn't be any issues with the App Store..

If the library was being released as sources, then licensing comes into play here and charging doesn't work that well..

If you are talking about libraries that will "improve" the iOS interface, you can forget about Apple allowing that in.

Without being a Member of the App Development Program, you are not allowed to see Apple's Licensing Agreement to the Dev's of iOS Apps, but i can give you a link, to Apple's Licensing Agreement from 2010, which was obtained under the Freedom of Information Act.
Have a read of the 28pgs and try and work it out yourself. Todays as in of May 2013 is 38pgs long, but all in all it basically the same. More pages because of the leap in Technology over the 3 yrs, and more the Dev's have to yes to, but the guts of it, is the same.
Enjoy

EDIT : After Re-Reading your post, this App might have to be in the App Store as a free App, but then with the Option of "In App Purchase's" that will then allow who ever bought the App access to *these libraries*, but without much more info than what you have given us, then its hard to determine how exactly this can be implemented in to the App then distributed.
And if this client is a Developer, then he/she/they should have some idea in how to implement these libraries, as well as to what the Apple's Licensing Agreement will say. If they make the App submit it, then if its declined because it doesn't follow Apples rule, they will have to find another way if implementation . . . .
What your asking is really something your Client, the Developer should be asking in Apples Dev Centre Member Area in the Developer Forums. All seems a bit fishy to me asking in a basic Mac Forum.