What is the meaning of Chetzi Eretz Bnei Amon?
Rashi: It is half of what [Yisrael] conquered from Bnei Amon
and from Sichon.
Yisrael took from Amon only what Sichon took from them! Surely Rashi refers to this; 'from Sichon' is what he owned before he took from Amon.