Today, most club players in Southern California are registered through US Soccer. Previously, most were registered through USYA (Calsouth). The Socal Soccer League, ECNL and the GA teams are all different gaming leagues, but all use US Soccer as their registar. CSL uses Calsouth.
Calsouth's transfer rule is simple: A player initiates the transfer through Calsouth's online system. The registrars of the two clubs approve or disapprove. No club can deny a transfer because of a financial dispute. In fact, there is almost no way to deny a transfer to a player unless the transfer is requested during the Fall roster-freeze period, which only last for about 3 months of the year. If the outgoing registrar does not respond to the request, the transfer is granted after a week.
The Socal Soccer League has taken transfer restriction to a new (and in my opinion, inappropriate) level. According to their rules, no player can ever transfer from one Socal Soccer League team to another without the outgoing DOC's approval. If a player is not paid "in full," then the DOC can block the transfer. If there is any dispute about whether the player is paid "in full," then the DOC can block the transfer. In scholarship cases, the DOC can block the transfer even if the player has paid every penny owed, and can add on additional fees and costs.