Seforno: Yosef was letting them know that his power extended to Eretz Kena'an too, and that even there he was able to have them killed. ¹

Ohr ha'Chayim #1: If you are lying; you may not hand over one of yourselves ² (for he will be killed), and all of you will die (refer to 42:19:151:1).

Ohr ha'Chayim #2: If you fulfill, I will give to you grain, and you will not starve.

Riva: Since they accepted to fulfill Yosef's command, the verse credits them as if they already fulfilled it.

Oznayim la'Torah: It means that despite the Halachah ¹ prohibiting ten men from handing over one of them if Nochrim demand that they hand over one of them or they will kill them all, they agreed; (a) on condition that nine of them are permitted to return home and one of them remains incarcerated - seeing as, since they would all be allowed to live, and even the one who they would pick would be no worse off than before, ² and (b) by accepting the offer, they would save their families from starvation.

Ha'amek Davar: They prepared to choose one to leave in prison - but it was difficult, for captivity can lead to death, and if Nochrim ask a group 'give us one of you; if not, will kill all of you,' they may not comply (Yerushalmi Terumos 8:4)

Malbim: Before, Yosef suggested bringing a good proof of their claim (they would send a Shali'ach from Egypt telling their brother to come). The current test (nine brothers will return, and come back with their brother) is not such a solid proof. Perhaps they will take a stranger; or they have many brothers at home (and therefore their father was willing to send 10)!