The Gemara in Yevamos (65a) proves from various sources that one may lie, or at least tell a half-truth, to preserve peace, at least regarding the past. (See here for more on that Gemara.)
Now, in all the scenarios presented there, the parties involved knew, or at least though, there would be strife otherwise. Would one be permitted to do so if you're unsure how the other party would react if you told the truth?