Untruth is an unwilling vice that is forbidden by the Islamic law. But, there are certain circumstances in which telling untruths becomes allowable. In case a vital interest cannot be achieved unless untruth is committed, the Islamic Sharia permits it. A s an example on such vital interests we cite the salvation of a Muslim individual from killing or capture, the protection of the honor of a Muslim, or the guard of his property. In such cases, untruth becomes obligatory, according to the Islamic law. Cor respondingly, when untruth becomes the only means for achieving a more important mission or remedial goal, it is then preponderant or allowed. Reconciliation
between people, the propitiation of wives, and trickeries of wars are matters for which telling untruth is allowable.
Imam as-Sadiq (a) said: “The teller of any untruth will be interrogated on a day –Day of Resurrection— except the untruths that are told in three situations: stratagems of war, reestablishing the ruptured relations between two parties—the reconciler, in this case, tells one party something that was not said about him by the other party and does the same thing with the other party so as to make peace between them—, and promises to the wives.
source : http://www.maaref-foundation.com