All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
If the matter is as you mentioned that you were not aware of the prohibition of purchasing such books without the permission of the concerned person, then Allaah willing, you are free from sin for purchasing this book. However, you may benefit from that book, and you are not obliged to purchase an original copy for that. But, in future you have to avoid repeating this sin.
Allaah Knows best.