Moral Dilemma-please help?

Ok, i bought a math textbook for about $130
When I finished the class, my fiance and my friend both needed it, so they were sharing the book.
My friend calls me and tells me he sold the book to the school buyback program for a measly $70
I had already promised to trade this book to someone else.
My friend is poor and doesnt really have money
The book is going online (used) for anywhere from 90–130
help

edit: my friend is not keeping the money…He said he was just trying to help me out. the question is what do I do?