Responsa for Bava Metzia 19:14
<big><strong>גמ׳</strong></big> אמר ריש לקיש משום אבא כהן ברדלא ארבע אמות של אדם קונות לו בכל מקום [מאי טעמא] תקינו רבנן דלא אתי לאנצויי אמר אביי מותיב ר' חייא בר יוסף פיאה אמר רבא מותיב ר' יעקב בר אידי נזיקין
— He [R. Nahman] answered him: A labourer is different, as his hand is like the hand of his employer.<span class="x" onmousemove="('comment',' The employer's right to the object found by his employee has nothing to do with the question whether one may acquire an object for a neighbour, as in the case of the employer the reason why he is entitled to the object found by his employee is that during the time of the employment the employee belongs to the employer, and anything that the former acquires during that time belongs to the latter. ');"><sup>14</sup></span>
Teshuvot Maharam
A. A must pay B the money coming to B from the Gentile for the following reasons: a) We assume that A became a trustee for B's part of the loan and, therefore, must collect it for B, for how else did B expect to get his money back?; b) A caused a definite loss of money to B and must compensate him for such loss; c) the Gentile became obligated to pay money to B, even though he did not know that part of the money loaned to him belonged to B. Therefore, when the Gentile paid off his debt to A, he gave to A the money he owed to B, and A must give that money to B.
SOURCES: Pr. 254.