Feel free to compensate
Yes, the Entypo Pictogram Suite is absolutely free. Even for commercial use. But if you or your client still want to make a contribution to the past and future development of Entypo you are more than welcome to do so. However, 50% of all contributions will be donated to Wikipedia.
I would also be very happy if you would send me examples on how you have used Entypo in your design. In time I will try to sort out a gallery section showcasing the best and most interesting executions.
In both cases just drop me an e-mail.