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The Posuk says that Moshe told Pharaoh that Hashem is going to bring the final plague, the death of the firstborn, כחצת הלילה - at around midnight.
Rashi says the reason Moshe changed what 'ה had told him from "at midnight" to "around midnight", is because he didn't want Pharaoh's astrologers to say that he's lying & the plague didn't start at exactly midnight, since they don't know the exact time, as 'ה does.  
Why did Moshe feel it was so important that the astrologers wouldn't think he's lying, that he actually changed what 'ה had told him & reworded the warning?
The Posuk says that 'ה told Moshe to ask the Jews to go around to the Egyptians & ask them for their gold & silver & he uses the words דבר נא "speak please".
Rashi says the word נא denotes a request, & the reason he's asking is because he didn't want Avraham Avinu to have any grievance against him that he fulfilled his promise of "I'll enslave them", but the promise of "I'll send them out with great wealth", he didn't. So 'ה told Moshe, as a favor to me, please have them ask for gold & silver so they'll go out with great wealth & Avraham won't be upset.
The obvious question is, shouldn't 'ה keep his promise just because he promised & not just so that Avraham won't be upset?Also, when Rashi brings this down he calls Avraham אותו צדיק. Why refer to him as a Tzaddik now?
We could answer like this. Of course 'ה was going to keep his promise no matter what, & send them out with great wealth, but the great wealth that 'ה intended for them to get was the Torah, for there is no greater wealth than the wealth of the Torah.
The reason Avraham is referred to as a Tzaddik here is because he was worried that people wouldn't understand Hashem's intention & say that 'ה didn't keep his word. Therefore, Avraham wanted them to go out with real wealth so no one could have complaints against 'ה and say he didn't keep his word.
So too Moshe, even though he was telling the truth, if it could be interpreted otherwise, he had to change the wording so that no one should think he's not telling the truth. Just like Avraham wanted to make sure no one suspected 'ה of not telling the truth. 
We see from here how careful we have to be even when doing the right thing to make sure are actions or words aren't misconstrued & people only think the best of us!