After Bruce Ratner received permission this week to acquire Atlantic Yards land in pieces and delay payments to the MTA, Ratner now has to raise more than $500 million to build the Nets arena. Ratner still must get final state approval and overcome any new lawsuits, while selling about $586 million in arena bonds by Dec. 31 in order to qualify for tax-exempt status. The tax-exempt financing would reduce his borrowing costs by tens of millions of dollars. If he misses the Dec. 31 deadline, Ratner would have a short grace period to secure more expensive conventional financing, but most officials and bankers say that that is unlikely given the still frozen state of the credit markets. “In the event they are unable to secure financing under those terms,” said Gary Dellaverson, chief financial officer for the MTA, “the agreement is no longer valid and the MTA would have to decide what to do with the property.”