The Mordechai explains that in the case of the Mishnah the owner of the house did not have any reason at all to think that there was a treasure buried deep within the walls of his house. During the times of the Mishnah, finding buried Canaanite treasure was not nearly as common as during the period immediately after the Israelite conquest and settlement of the Land of Israel. However, if the case would have been that finding treasure within the walls of one’s home was a relatively common occurrence, then the owner of the house does acquire the treasure through kinyan chatzer since the owner of the chatzer does have a certain amount of knowledge regarding the treasure’s existence.
In our case it would follow that, according to the Mordechai, the man must give back the money to the thrift shop. Thrift shop owners know that people often inadvertently leave valuables in the things that are given to the thrift shop. Usually, a thorough search is done by the thrift shop on all the items brought to them before they place the merchandise on the store floor. (However, nothing is foolproof, and sometimes things like hundred dollar bills slip through.) The case of a thrift shop is not similar to the Mishnah, and the thrift shop owner is considered to have already acquired the money that had been forgotten in the suit through kinyan chatzer before the purchaser of the suit found the money. Thus, the man is required to return the money to the thrift shop owner. Of course, in a situation in which a thrift shop does not normally come across a certain type of forgotten valuable or does not find things in a certain type of merchandise, then that would be similar to the Mishnah and the customer would be allowed to keep whatever he found.