How could Eliezer eved Avraham commit "nichush"?
[See Sefer Hareidim 24:51]
2 P. Toldot:
How could Yaakov [ish emet] deceive his own father Yitzchaq?
Answer: In both cases they could rely on Prophecy for heter "b'Torat horo'at sha'ah"
1 Eliezer relied upon Avraham's guarantee:
"Hashem ...yishlach mala'acho ittach". Eliezer knew a Mal'ach was supervising him, so his Nichush was "kosher" only within that context
2 Yaakov was told by Rivkah. And Rivkah had been told by n'vu'ah ...sHein goyim..v'rav ya'avod tz'air. This prophecy enabled Rivkah to "force" the issue with a deception as a ho'or'at sho'oh in order to conform to Hashem's word