Midrash for Bava Metzia 63:5
<big><strong>מתני׳</strong></big> מצאה ברפת אין חייב בה ברה"ר חייב בה ואם היתה בבית הקברות לא יטמא לה אם אמר לו אביו היטמא או שאמר לו אל תחזיר לא ישמע לו
'What comparison is there?' he retorted. 'In that case, Seeing that money is being taken from one and given to another, a <i>Beth din</i> is needed;<span class="x" onmousemove="('comment',' It is so regarded because the Mishnah states that actually he is only entitled to the pay of an unemployed worker, hence, when he stipulates that he is to receive more, and the stipulation is allowed, it is the equivalent of taking money from one and giving it another. — The power of a Beth din to do this is based on the principle, hefker by Beth din is hefker, i.e., Beth din is empowered to abrogate a person's rights in his own property, and declare it ownerless; therefore the court can also take from one and give to another, ');"><sup>5</sup></span>