Who is to get the reward on the giving of Tzedaka, the giver or the receiver? And please, does it depend on the mood of the giver for some spiritual benefit to be granted?
The Shulchan Aruch Yoreh De'a 247 says that the giver receives great reward.
The giver's mood does make a difference. Shulchan Aruch (Yoreh De'ah 249:3) says (paraphrased and shortened from Rambam, Hil. Matnos Aniyim 10:4):
צריך ליתן הצדקה בסבר פנים יפות, בשמחה ובטוב לבב, ומתאונן עם העני בצערו ומדבר לו דברי תנחומין. ואם נתנה בפנים זועפות ורעות, הפסיד זכותו.
"One must give tzedakah with a pleasant facial expression, with joy and goodness of heart, commiserating with the poor person's troubles and speaking words of comfort to him. If one gives [tzedakah] with an angry and mean countenance, he loses his merit."
Further on (249:13) he says (also from Rambam, ibid. 10:14) that the lowest level of charity is giving unwillingly (which implies that this is a valid, though sub-optimal, level of tzedakah). Shach there (:9) points out that the case here is that the giver is indeed giving against his will, but at least puts on a pleasant face while doing so.
The Ramban states in Devorim (Perek Kof Beis, Pasuk Vov) that "there is no benefit in our keeping of mitzvos to the Holy One blessed be He." One cannot "benefit Him" or "harm Him" through the keeping or not keeping of mitzvos. He states further that our "words of praise and remembrances of his miracles are considered as nothingness and emptiness to him. All these [mitzvos] are for our benefit alone. This is something agreed upon by all our Rabbis."
So from the Ramban one can clearly see that the performer of any Mitzvah will be benefited from doing the Mitzvah. The commandments are designed to help a person develop a correct relationship to Hashem. To put in another way, each Mitzvah deals with an aspect of a persons personality and helps move the person to be in line with what is real and true. Moving the person from their illusions and fantasies to a more clearer picture of reality. This of course will help a person have a clearer and proper ideas of Hashem to the extent man can have knowledge of Hashem. This of course is one approach to Mitzvot, however, I think it is important to clarify what you mean by "spiritual benefit" in terms your mind can truly understand. Giving Tzedakah will certainly help a person have a proper relationship to his money and his possessions. It can help a person recognize that money is merely a means, not an end in itself. A person may recognize that his possessions are a means by which he can assist in his development and relationship to Hashem. Furthermore, it forces a person to recognize the needs of others outside himself. In order to relate to Hashem and an objective reality outside oneself one must be able to consider someone besides his own narcissistic subjective reality.
The main thrust of my approach is to suggest ones internal attitude and philosophy is what is vital to the performance of the commandment. A monkey can pick up a Lulav, what differentiates us from the animal world is our ability to think and understand the ideas and perfection of the commandments. An approach for you to consider.
I believe the sources quoted above by the Rambam and Shulchan Aruch address the Legal aspect of your question.