The gemara (Baba Kamma 69b) writes that one who steals from a thief is exempt from paying keifel because keifel is obligatory only where "v'gunav m'beis ha'ish", when property is taken from its owner, not where property is taken from a thief.
Does the gemara mean that one is not labelled a thief for taking stolen property (Robin Hood?), or does the gemara mean that one is a thief but there is simply no penalty of keifel?
The Rama (C.M. 34:7) paskens that one who divides stolen goods with a thief is not disqualified as a witnesses. The Tumim asks why this should be so -- why is witholding stolen goods any different than denying possession of a pikadon left in one's care? Witholding a pikadon is viewed as theft and disqualifies the person from being an eid; shouldn't the same be true of witholding stolen property?
The Ketzos answers that there is a basic distinction between these cases. The pikadon left to be watched is considered in its owner's possession until the watchman falsely claims it as his own. It is the moment of denial which transforms the pikadon into stolen goods. However, in the case of splitting booty with a thief, the goods are stolen property once the thief makes off with them; the splitting of the spoils adds nothing to the crime of theft which has already been perpetrated. "V'gunav m'beis ha'ish" is not just a dispensation from keifel, but teaches that there is no crime in taking stolen goods. Therfore, splitting the booty does not disqualify one as an eid. (See the Nesivos who disagrees).