Back to Chapter 23 about deceit
Translated by Ralph Anzarouth and an anonymous friend
We know that truth is telling things as they really are without any falsehood, whether in relating events to others or replying to their questions, either by way of testimony or in every other way. The virtue of truth is even more refined among the righteous, as is told by our Sages of blessed memory in relation to (Psalms 15, 2) "one should speak the truth in one's heart": here they bring the example of Rav Safra (Talmud Bavli, treatise Makkot 24a), where he had decided in his heart to sell a certain item for five dinars, and when the purchasers returned offering him ten dinars he would not agree to take more than five dinars in order to fulfill what is written (ibid.) "one should speak the truth in one's heart", in other words even if it had not yet passed his lips1.
The virtue of truth can be compared to the children of the bird called the partridge. When one of this species lays its eggs, another bird comes, takes them and hatches them. After the chicks have hatched, nature teaches them to recognize the voice of the real mother, so that as soon as they hear their real mother's voice, they go to her and leave the other. Such is the truth, that it ultimately reveals itself, as it is written (Psalms 85, 12): "Truth sprouts from the earth". King David further said (Psalms 51, 8): "You desire truth in my inner self". His son king Solomon said (Proverbs 12, 19): "Truth is eternal etc.". He further said (Proverbs 3, 3): "May kindness and truth never leave you". Our Sages of blessed memory said (Talmud Bavli, treatise Shabbat 55a): "Rabbi Chanina said: ’The seal of the Holy One, blessed be He, is truth2.’ " And He is G-d of truth and desires only men of truth, as it is written (Psalms 101, 6): "My gaze is concentrated on the faithful men of the earth, that they should sit with Me; the one who behaves with integrity will be My servant". Aristotle said that one who wants to conceal the truth or to cover up a lie will have to toil hard. Cato said that if you promised one thing to one person, you should not promise it again to another.
It is also written that there was once a very rich man who gave away all his money to Tzedakkah. Then he went with some ascetics into the wilderness to serve Hashem. One day his friends told him and another person to go to the city to sell two donkeys which because of their age were no longer fit for carrying burdens. He went to the city market where people came looking to buy and they asked him if these were good donkeys. He replied: "Do you think we would be selling them, if they were still fit for use?" They asked him further why there were bare patches on the donkeys' backs and tails. He replied: "Since they are very old, they stagger under their burdens and we pull them by their tails and beat their backs and in this way they have bare patches". When they returned to the friends and the donkeys were still with them, his fellow traveler related them all that had happened and why he had not sold the donkeys. They called him and asked him: "Is it proper for a seller to belittle his merchandise3 ? He replied: "Do you think that I left my home and relinquished my fortune to be able to dedicate myself to telling lies, in order to sell decrepit donkeys? How many donkeys, camels, cattle and livestock did I give away in order to serve my Creator, that I should now betray my faith?" On hearing this, they were so shocked that they were ashamed to pursue the subject further.
Notes of the translators:
 The verse uses the expression "in one's heart", therefore the righteous adhere to what they have decided in their heart even though they have not yet spoken about it and even if this decision causes them a financial loss.
 Which means that everything which pertains to Hashem must be perfectly true.
 This question hints to the propensity of many vendors to always praise their goods, whereas many prospective buyers will always display lack of interest in order to make a better deal (see Proverbs 20, 14).
The whole book Tzemach Tzadik in Hebrew (printed in Rashi characters) can be downloaded in PDF format and read online at Hebrewbooks.org. Chapter 24 about truth is found at pages 50-52.