I am just preparing this sugya to teach to me dear talmidim in school, so I hope I can help out here.
There are halachos in the torah of a persons responsibilty if a mishap happens to an article whilst it is in the care of another person. The first stage is a person who is unpaid for his service, with the second being one who is paid. The one who is paid obviously has more responsibilties than the one who is doing it out of the goodness of his heart.
The gemara brings a machloikes regarding the status of a Shoimer avaida (a guardian of a found item before he has an opportunity to return it to its rightful owner).
Rabba says that he has the status of an unpaid guardian, seeing as though he never gets paid for what he is doing.
Rav Yoisef says that he is like a paid guardian, as he is being paid for his services. The gemara explains that when a person is occupied with a mitzva he is exempt from doing another unrelated mitzva. Therefore whilst the guardian is actively occupied with caring for the found item, he is exempt from answering the door to poor people. This amount that he saves by not having to answer the door and help the poor person, is his gain, making him like someone who is paid for his services.
This “gained” money is called “P’ruta D’rav Yoisef” as it is the p’ruta that he gained in Rav Yoisefs opinion.
For more info. see gemara Bava Kama 56b.
Sorry for being so long winded.
I hope this helps