Pirkei Avos describes the main differences between Bil'am and Avrohom
Avinu. Rashi mentions or alludes to Avrohom many, many times in his
explanations about Bil'am in our Parsha, usually drawing distinction
between the two, and always to Avrohom's advantage.
In addition there is a striking parallel/distinction between Bil'am and
Avrohom which can be gleaned from Rashi, pointing yet again to the subtle
beauty of every word of Rashi.
The pasuk says that Bil'am had two young men with him (22:22). Rashi says
this teaches that an important person "should" take two other people
(meaning not just one) along when he travels to serve him; this way if one
of them needs to excuse himself the other "can fill in for the person who
is temporarily indisposed".
Similar, but different: The pasuk in Parshas Vayera (22:3) says that
Avrohom took two young men with him on the way to the Akeida. Rashi says
that this teaches that an important person "may not" travel without two
other people along to serve him; this way if one of them needs to excuse
himself the other "will still be with" the important person.
These differences are explained in many different ways by the M'forshei
Rashi. My favorite explanation (see Be'er BaSadeh and Gur Aryeh) is based
upon the gemoro in Brachos which says that a talmid chochom may not travel
at night alone. Avrohom was a talmid chochom, and therefore he was not
permitted to travel alone (as the travel included overnights); he must
therefore take two people so that if one must excuse himself the talmid
chochom will still be accompanied by the other person. The key is that he
not be alone.
Bil'am was important, but not a talmid chochom. For an important person
who is not a talmid chochom there is no violation if he goes alone, it is
simply proper behavior (derech eretz) for him to have someone else along
to serve at all times. He takes two people so that if one becomes
indisposed the other will still be able to provide the service.
Rashi is explaining that we derive the stricter prohibition about a talmid
chochom from Avrohom while we derive the derech eretz about a regular
important person from Bil'am.