there should be no problem;
the OP needs to transfer 80k from his Amex to his FB acct (till 2/29 theres a 25% bonus so 80k = 100k) then he already has the JFK-PRG so then he should beable to grab the DL flights on 8/6 that he said was OK
the only thing I dont know is how long does it take for the Amex transfer to get into the acct and will FB hold the seats in the mean time?