What does Dumi write in his letter to his family?

In Journey to Jo'burg by Beverly Naidoo, Dumi is the older brother of Grace, a fellow traveler Naledi meets on the way to Johannesburg. Dumi was a freedom fighter who actively engaged in protests against the oppressive apartheid system in South Africa. This made Dumi—a young, black, politically active male—a great threat to the police and to the government as a whole.
At one point, Dumi is detained by police officers and then physically assaulted while in custody. Afterward, Dumi disappears, and his family fears he has been killed. Forced disappearance was common in South Africa during the apartheid era. High-profile revolutionaries and student activists were equally highly targeted by the military and police.
Later on, however, Grace and her family receive a letter from Dumi. He explains that he left South Africa after being released and went to another country in order to study. Dumi states that he is studying in order to come back to South Africa armed with knowledge and new skills so that he may continue fighting for liberation against the European colonialists.