Tzedaka

שמות ל ,יב: כִּי תִּשָא אֶת־רֹאש בְׁנֵּי־יִּשְׁרָאֵּל לִּפְׁקֻדֵּיהֶם וְנָּתְנוַּ אִּיש כֹפֶר נַפְׁשוֹ לַיהוָֹה בִּפְׁקֹד אֹתָם וְׁלֹא־יִּהְׁיֶה בָהֶם נֶגֶף בִּפְׁקֹד אֹתָם

The Baal HaTurim notes that word ונתנו (and they give) can be read in either direction (right to left or left to right). This teaches us that when a person gives Tzedaka for a holy purpose (such as for the Mishkan) in the end it would return to that person for a benefit and he would not be missing anything from doing this Mitzvah.