The Championship side are banking on collecting a loan fee from City this month to pay their players and staff their wages.
The 35-year-old goalkeeper is with City as third choice, and City wanted to secure him permanently this month.
And according to the Daily Mail, Derby need the £500,000 they will be owed for the transfer to fulfill their financial obligations.
Some of the fee has been paid, but the rest of it may be due this month.
City are unhappy with Derby, as they believe they should be allowed to sign Carson permanently for a modest fee given his age and contract length with Derby.