Chasidut for Shabbat 237:6
בעא מיניה רבי מר' ישמעאל ברבי יוסי עשירים שבא"י במה הן זוכין א"ל בשביל שמעשרין שנאמר (דברים יד, כב) עשר תעשר עשר בשביל שתתעשר שבבבל במה הן זוכין א"ל בשביל שמכבדין את התורה
— [So] he went, sold all his property, and bought a precious stone with the proceeds, which he set in his turban. As he was crossing a bridge the wind blew it off and cast it into the water, [and] a fish swallowed it. [Subsequently] it [the fish] was hauled up and brought [to market] on the Sabbath eve towards sunset. 'Who will buy now?' cried they. 'Go and take them to Joseph-who-honours-the-Sabbaths,' they were told, 'as he is accustomed to buy.' So they took it to him. He bought it, opened it, found the jewel therein, and sold it for thirteen roomfuls<span class="x" onmousemove="('comment',' R. Tam translates: vessels. ');"><sup>19</sup></span> of gold <i>denarii</i>.<span class="x" onmousemove="('comment',' This, of course is an exaggeration, and merely implies much money, 'thirteen' often being used figuratively in that sense, cf. supra p. 585, n. 6; Hul. 95b (Rashi). ');"><sup>20</sup></span> A certain old man met him [and] said, 'He who lends to the Sabbath,<span class="x" onmousemove="('comment',' I.e., expends money in its honour. ');"><sup>21</sup></span> the Sabbath repays him.'
Kedushat Levi
The Talmud, Shabbat 119 in commenting on the repetition of the words עשר תעשר, says: “give your tithe so that in turn you will become wealthy, i.e. able to tithe more.”
In order to understand what appears at first glance as an arbitrary way of interpreting this verse, we must consider the following. When a person makes a charitable donation, either to an individual or a charitable organization, this deed comes to the attention of G’d in heaven. Let us illustrate by means of a parable. When a person has donated a gold coin to charity or some other measurable unit of his harvest, a tenth of this rises heavenwards, and when it arrives there it is inspected and its origin is identified. It is recognized that this donation was the result of the donor possessing tenfold the amount that he donated to charity. As a result of having been associated in heaven as belonging to the donor of the gold coin that reached heaven, the remaining 9 gold coins belonging to the donor on earth are now considered in heaven as being part of the fulfillment of this commandment. The reason is simply, that unless the donor had owned ten gold coins he could not have donated this coin to this particular charity. As a result, the donor will be enabled to donate ten gold coins as his tithe from the following year’s harvest (income). It follows that his entire harvest of the previous year had by then been converted into being only a “tithe,” i.e.10% of his harvest (income). This process is liable to repeat itself year by year, as long as the owner of the gold or field keeps up the mitzvah of tithing meticulously. The Torah by writing עשר תעשר, hints that people performing this commandment will experience that their wealth increases tenfold i.e. עשר תעשר is equivalent to saying: “ten times ten.”.