פירוש על קידושין 38:17

R. Eliezer reads another verse as allowing her to be sold to those unfit for her. So then what does he do with the word “to be a slave”? He claims that it teaches that he can sell her to relatives, even though these relatives cannot marry her because that would be incest.
Again, the baraita claims that this is obvious—if he can sell her to those unfit for her, clearly he can sell her to relatives. But this argument is easily debunked. He can sell her to those unfit for her, because even though designation would be prohibited, it would still be possible. But we might have thought that he cannot sell her to relatives because these relatives could not designate her. Therefore, the verse teaches us that he can even sell her to a relative, one who could not designate her because marriage with this relative is prohibited.