http://revach.net/article.php?id=2713

Section: Moadim   Category: Around The Year
Chodesh Elul: Bnei Yisaschar - A Month of Hashem's Charity

The order of the letters of Hashem's name for Elul is Hey, Hey, Vuv, and Yud.  These letter appear in the Sofei Teivos (last letter of each word) of the words (Devarim 6:25), "U'Tzedaka Tihyeh Lanu Ki", and it will be to our credit.  The Bnei Yisaschar says that the gematria of the letters of these words without the Sofei Teivos has the same numerical value as the word B'Tshuva (715).  This teaches us that the name of Hashem identified with this month makes it a special time to do tshuva and return to Hashem.  Furthermore the Roshei Teivos (Vuv, Tuf, Lamed, Kuf) has the same gematria as the words Tammuz Av.  This shows that the harsh judgement hovering over the months of Tammuz and Av are sweetened in Elul.   

Additionally, the Bnei Yisaschar brings form the Rishonim that if you take the letters of these four words, excluding the first and last letter of each word, it spells the words "Hin Tzedek".  The Remez here is that to do Tshuva you must be very careful with all that you say to make sure no untruth, however minute, creeps into it.  Moreover the gematria of the Roshei Teivos (456) is equivalent to Avrohom Yitzchok.  These to Avos form the outer bounds with Ahava and Yirah, respectively.  While Yaakov with the Midah of Emes, Hin Tzedek (the middle letters) fits right in the middle between his illustrious father and grandfather.  

If we do Tshuva from the bottom of our hearts with real truthfulness the month of Elul will be the greatest Tzedaka we ever received.