The quotation from the text that best shapes the theme that true friends make sacrifices for one another is "I will give myself up as a pledge of my friend's return. and if any accident befalls him I will die in his stead".
This quotation is taken from Damon and Pythias, a story which illustrates the value of a true friendship. In this myth, Pythias, who is Damon's friend, is accused of betraying Dionysius. Phythias asks the ruler to allow him to go home to settle his affairs and then return to be executed. Dionysius only accepts his petition when Damon offers to take his friend's place until he comes back. This quote shows that Damon is willing to give up his life for his friend. When Pythias actually returns, Dionysius is so amazed by Pythias and Damon's friendship that he decides to pardon both of them.