In this shiur a question was raised by someone in attendance as to why Hashem's "complaint" to Avrohom about Sarah's lack of emunah did not amount to lashon ha'ra. The Rosh Yeshiva answered from Rav Moshe Feinstein (based on a Gomorrah in Pesachim) that Hashem's "complaint" was la'to'eles in that it was meant to educate Sarah. The Rosh Yeshiva said it was analogous to it being acceptable for someone to complain to a rebbe about his talmidim if this would enable the rebbe to re-direct his students. The Rosh Yeshiva then cited the example of Yoseph's complaints to Yaakov about his brother’s apparent misdemeanours. What I don't understand is that Yoseph’s reports to his father were regarded as lashon ha’ra, for which he was punished. Rashi explains how 3 negative incidents later occurred to Yoseph mida k'neged mida as a result of these reports?