פירוש על קידושין 39:5

At the end of last week’s daf we saw a dispute between the rabbis and R. Eliezer—the former hold that he may not sell her to his relative, while the latter holds that he may. So the second baraita agrees with the rabbis, but with whom does the first baraita, the one that distinguishes between the father and the son, agree?

The baraita that allows a man to sell her to his father can even agree with the rabbis who do not allow her to be sold to relatives. If a man sells his daughter to his father, the father can designate her to his other son, who is her uncle. Uncle-niece marriage is allowed according to Jewish law. Thus he may sell her to his father because designation is possible. This would not be true when it comes to selling her to his son, because she is not allowed to marry her nephew.