Kiddushin, Chapter Two, Mishnah Ten
In this mishnah we learn about a man who betroths a woman using certain things from which it is not prohibited to derive benefit.
If he betroths with terumot, tithes, priestly gifts, the water of purification or the ashes of purificationbehold she is betrothed, even if he is an Israelite.
Terumot: Terumah can only be eaten by a priest. A priest can use terumah for betrothal and then the woman may sell it. However, even an Israelite can potentially own terumah. For instance, if someones maternal grandfather is a priest, he is not a priest because the priesthood is not inherited through his mother. In such a case he will inherit from his grandfather, if his mother inherits from her father and then dies. The non-priest cannot eat the terumah which he inherits, but he can sell it. He could also use it for betrothal and then the woman can sell it. He would have to tell her that it is terumah, because terumah is less valuable than regular food.
Tithes: These are given to the Levite, who may use them for betrothal. An Israelite can use them for betrothal in the same way described above.
Priestly gifts: This refers to parts of non-sacred animals given to priests (see Deuteronomy 18:3). The priest can use them as betrothal money and if they come into the hands of an Israelite, he too can use them.
The water and ash of purification: To purify someone who came into contact with a dead body, they would burn the red heifer and put its ash into water. According to the Talmud, our mishnah refers to someone who betroths with payment he received for drawing the water or for bringing the dust. One cannot betroth with the water or ahs itself because there is no financial benefit to be derived from them.
I should note that I have explained that an Israelite cannot betroth with terumot or tithes that he separates from his own produce. Such gifts must be given for free directly to a priest or Levite. However, it is possible to explain that the mishnah is referring to the tithes or terumot that an Israelite himself separates from his produce. The Israelite has the benefit of being able to give such gifts to whichever priest or Levite he so desires. This benefit is worth moneyfor it will make the priest or Levite look favorably upon him. It is with this benefit that he is betrothing the woman. She now has the benefit of giving the terumot or tithes to anyone she wishes. While this may be a small benefit, remember, it only takes a perutah.