Ta’anis 4 relates that three people did not formulate their requests properly; in two of the cases things worked out anyway, and in one case it didn’t. The second case the gemara presents is Shaul’s promise to give his daughter to whomever killed Golyas without considering that a mamzer or eved might be the one to complete the task. The first case the gemara presents is Eliezer’s intent to give Yitzchak to whichever girl would draw water for him and his camels without thinking that the girl who does so may be blind or lame.
Tosfos asks why the gemara is concerned lest Shaul’s daughter be promised to an eved or mamzer but mentions only concern for physical features with respect to whom Yitzchak would marry. Tosfos answers in the name of Rashi that Yitzchak lived pre-mattan Torah and the issur of marrying a shifcha or mamzeres did not yet apply.
Rashi’s assumption seems to be that even with respect to laws of marriage the Avos behaved as bnei noach and not as if commanded in mitzvos, an assumption that seems slightly odd considering that Avraham specifically wanted a girl from his own family for Yitzchak. Remember as well that Rashi on chumash writes that Eliezer himself had a daughter which Avraham turned down as a match because Eliezer was an eved. The Maharatz Chiyus on the daf suggests that Rashi's chiddush might be true only with respect to marriage, as we see from Ya'akov marrying two sisters - an interesting idea, but it begs the question why. Be that as it may, I was very surprised that when I checked the index to the Parashas Derachim thay I have and thumbed through the first two derashos I could not find this Tosfos quoted in his discussion of the status of the Avos pre-mattan Torah.
A side point: Eliezer’s promise to take the girl who drew water first was stated in the context of a tefilah – he was asking Hashem for help in determining the right match. Even though his intentions were proper, since in formulating his tefilah he left open the possibility of a bad match occurring the gemara considers this an improper request. Just interesting that the particular words of tefilah carry such significance above and beyond the thoughts of one’s heart.