Understanding that lying, intentionally saying that which is known to be not true, is (with certain exceptions) as unacceptable in Islam [as in Christianity]. Are Muslims required to be honest (open and disallowing false impressions) in their dealings with others, or is it permissible to allow others to have such impressions which are known to be untrue?
As an Islamic rule, it is considered as a Wajib (mandatory) practice for Muslims to be veracious. Accordingly all Muslims ought to be considered as truthful people.
According to the holy Qur'an:
"This is the Day when the truthful will benefit from their truthfulness." For them are gardens [in Paradise] beneath which rivers flow, wherein they will abide forever, Allah being pleased with them, and they with Him. That is the great attainment. (119)
قَالَ اللَّـهُ هَـٰذَا يَوْمُ يَنفَعُ الصَّادِقِينَ صِدْقُهُمْ ۚ لَهُمْ جَنَّاتٌ تَجْرِي مِن تَحْتِهَا الْأَنْهَارُ خَالِدِينَ فِيهَا أَبَدًا ۚ رَّضِيَ اللَّـهُ عَنْهُمْ وَرَضُوا عَنْهُ ۚ ذَٰلِكَ الْفَوْزُ الْعَظِيمُ ﴿١١٩
So we see that Allah has mentioned in the Holy Quran that “today is the day that being a truthful person would be profitable for them and …
Or for instance, it has narrated from Imam Ali (peace be upon him) that Said:
Truthfulness and honesty are the best instruments for salvation from the calamities of the world and Akherah (doomsday).
غررالحکم ج 1 فصل اول ح 11 . / . Qorar-al-Hkam, Vol.1, chapter1, 11)
Or according to another tradition, it has narrated that: Al-Sedq (honesty) is deliverance (salvation). (Qorar-al-Hkam, Vol.1,120). غررالحکم ج 1 فصل اول ح 120
In regard to the lying, there are many traditions regarding that, even there are some verses in the holy Quran that mentioned regarding lying, such as: That holy Quran considers the lying as the main reason of Bid'ahs and so on (Nahl:116)
On the whole, it is considered as a negative act which Muslims ought to observe it. And as you mentioned, Moslems ought to be honest in their dealings with others.