Q: What payment is required to render a guardian a shomer sachar (paid guardian)?

A: Any monetary benefit the guardian receives in return for guarding an item renders him a shomer sachar. Therefore, if he received even as little as a prutah (a few cents), he is considered a shomer sachar (Shach 303:1).

Furthermore, even if the guardian received no actual payment but will be repaid in kind - e.g. “You watch for me today and I’ll watch for you tomorrow” - he is considered a shomer sachar. Therefore, a group of friends or mothers who take turns watching for each other are all considered shomrei sachar (C.M. 305:6).

Similarly, any favor that is agreed upon in return for watching - even lending the guardian an item or picking up something for him - is considered payment that renders him a shomer sachar (Aruch Hashulchan 303:3).

If a person was paid to watch for a certain number of days, but the item remained with him afterward, he is considered a shomer sachar for the days for which he was paid and a shomer chinam for the days afterward (304:6).