Midrasch zu Dewarim 24:3

3) "and redeemed she has not been redeemed": through money or its equivalent. Whence is (the possibility of redemption by) writ (of emancipation) derived? From "or freedom not given to her," and elsewhere (Devarim 24:3): "And he shall write to her a writ of divorce" — Just as there, a writ; here, too, a writ. This tells me only that money effects half- (redemption [viz., "and redeemed she has not been redeemed"]), and that a writ (of emancipation) effects complete (redemption). Whence is it derived that a writ, too, can effect half-redemption? From "and redeemed she has not been redeemed, or freedom not been given to her" — Just as money can effect half- (redemption), so, a writ.