Am I sinful for giving away half a book that was requested of me, when I had forgotten that the teacher entrusted me not to give it to anyone?

One of the Sharia principles is that error and forgetfulness remove sin. The Quran explicitly pardons them in the verses: {Our Lord, do not hold us accountable if we forget or make a mistake} and {And there is no blame upon you for that in which you have erred, but [only for] what your hearts have intended}. Therefore, if you revealed the secret out of forgetfulness, there is no blame upon you.