From ESPN's Andy Katz:

Sources said Biyombo's name was on a list sent out by the NBA Wednesday detailing players who were subject to a buyout. Teams have investigated and concluded that Biyombo would owe roughly $1.5 million, possibly equal to his NBA rookie contract, to get out of his deal with Spanish team Fuenlabrada.

Sources said the buyout figure could be negotiated, but the matter also could end up in court.

Under the current CBA, NBA teams are restricted from contributing more than $500,000 to help a player buyout a contract. That said, the current CBA expires July 1, so by the time Biyombo actually signs his first NBA contract the rules may be different.