1)
Why does it say "Chinam Nimkartem"?
1.
Rashi: For matters of Chinam, the Yetzer ha'Ra - you get no reward [for obeying it].
2.
Radak: You were not sold for money, rather, due to sins.
3.
Malbim: This explains why He can command them to leave Galus by themselves. Do not say that a sold slave cannot leave without his master's consent, and without redemption money.
2)
Why does it say "v'Lo v'Chesef Tiga'elu"?
1.
Rashi: You will not be redeemed via money, rather, via Teshuvah.
