At the beginning of this week's Parsha, the Torah tells us that donations should be taken from every man whose heart desires to make Hashem an offering (שמות כ"ה:ב'). This instruction follows all of the many laws regarding monetary matters described in last week's Parsha. Parshas Terumah follows Parshas Mishpatim in order to teach us an important lesson. Parshas Mishpatim teaches us that one has to use one's money in accordance with the law; Parshas Terumah deals with donations to charity. If someone gives charity by stealing from others then it is not considered charity. One can't do a Mitzvah by means of an Aveirah. Only after the Torah makes it clear that one must be honest in making money can it discuss how one can donate one's money.
Many commentators wonder why the above Posuk says "ויקחו לי תרומה", meaning "take me an offering," using the word "take" to refer to the donations, even though the people were giving and not taking. The Beis HaLevi explains that this is because only the money that a person gives to charity is considered really his. In general, the money which a person has is not really his; it does not become part of him. Rather, it is something which he may use while he is here on this Earth. Only by giving charity does one demonstrate that one's money is his own. Therefore, the Torah uses the term "take" when it refers to giving to worthwhile causes because in actuality, he is "taking," the money for himself because only that money that he gives to charity becomes truly his own.
Nevertheless, Rashi notes that the donations should be given only for Hashem's sake. Why does Rashi stress this? What is the difference what the person's intentions are when he performs this Mitzvah? Rabbi Leib Chasman that one should focus only on the giving, and not on what he may receive. He illustrates this with an example of a person who delivers milk every morning regardless of the weather so that the children can have milk for breakfast. One would certainly say that this person has a very high level of kindness. But if one hears that this person gets paid for this, one would conclude that the person is just a regular milkman. Therefore, one should do things out of kindness alone and not worry at all about what he is going to get for it, even if he will be receiving something substantial.