Reference for Bava Metzia 147:3

אמר רב פפי משמיה דרבא האי סיטומתא קניא למאי הלכתא רב חביבא אמר למקניא ממש

This is so only if he [the agent] did not make up a separate sealed package of each man's money; but if he did, then for whom he has bought, he has bought, and for whom he has not bought, he has not bought.

Jastrow