The passuk states ואיש את קדשיו לו יהיו “and a man, his holy things will be to him.” What does this mean?
Rashi brings from the Midrash Aggadah: One who holds back from giving his ma’asros (tithes)לו יהיו, to him will be the ma’asros. This means that in the end his field will produce a tenth of its annual yielding.
Tosfos brings a story to express this lesson: there was a wealthy farmer who owned a field which yielded one thousand kur annually. He would take off ma’asros of one hundred kur each and every year. When he was sick and about to die he called his son and told him “the field’s annual production is one thousand kur, and make sure to tithe the field.” The man died and his son took over. The field once again produced one thousand kur that year which he tithed one hundred kur as his father warned. During the second year the son thought it is a huge loss to separate a tenth, and declared that he won’t give a tenth. That year the field yielded a total of one hundred kur, a tenth of the field’s annual production. The Midrash Tanchuma explains that each year the field yielded one hundred less until it only produced one hundred kur. The son’s relatives heard what happened and they came rejoicing, dressed with fancy clothes. The son said, “it seems you are happy with my downfall.” They responded, “We are in pain for you, yet it is your fault that this occurred; you brought it on yourself because you didn’t separate ma’aser. When you received the field you were the master and Hashem was the Kohen since the tenth was His portion. Now Hashem is the Master and you are the Kohen, since you only received a tenth. The field which originally produced one thousand kur, Hashem took and gave to you one hundred kur. The passuk says לו יהיו “to him will be the maser” if he will hold back from separating.
The Nefesh HaGer adds that Onkelos adds in translating קדשיו “ma’aser” hinting to this Midrash Aggadah!
In Megilas Rus it mentions that Boaz took care of Rus by feeding her extra, giving her to drink, and telling the youths to enable her to collect more than others. When Rus tells Naomi this, the passuk states ותאמר שם האיש אשר עשיתי עמו היום בועז “and she said ‘the name of the man which I did with him today, is Boaz.’” Instead of stating אשר עשה עמי “that which he did with me,” it states “that which I did with him”. The Taklin Chaditin explains that we derive from here: more than a rich person does for a poor person, the poor person does with the rich person, by giving him the opportunity to receive a zechus.
We specifically remember this lesson on the Yom Tov of Shavuos, at the time when we receive the Torah and mitzvos. This is as Rabi Chanina ben Akashya said, “Hashem wanted to be mezakeh Yisroel, and therefore gave them an abundance of Torah and mitzvos.” How fortunate are we to have so many opportunities which are mezakeh us!