Official Amendment to Franchise GM Bidding Rules...
Upon winning a bid that sets a team over the cap, the GM is granted 24 hours to post the transactions that will make the cap work (either a trade or releasing players). If the transactions are not valid or not posted within 24 hours of the bids being finalized, that team no longer receives the player. If this is a first time offense, the GM is given a warning. For all subsequent offenses, the team is penalized a 10% cap penalty.
After such a failed bid, the player goes back to the bidding board, leaving off at the highest bid prior to the winning bid, and the 72 hour clock is restarted. The offending team can no longer bid on that player, even if they clear up the cap space to do so.
No exceptions are granted to teams that won the bid but had a trade fall through - if you're not certain you'll have room, make the trade first. In other words, don't bid on a player you can't afford! Note that this applies only to FA. No trades can be completed that set a team over the cap - the trade must be re-worked.