Fannie Mae has signed a definitive agreement to buy $10.3 billion of mostly delinquent mortgage servicing rights from Citigroup for an undisclosed sum. Roughly 64,000 Fannie-backed loans are involved in the transaction. The GSE confirmed the sale last week. It plans to transfer the receivables to one of its specialty servicers, but declined to say which one. It has about five, including the IBM-owned Seterus and Walter Investment.