Bonus money for IFAs comes from prospect payroll. So to get to over 20m you need to transfer from player or coach salary.
Bonus money for ML FAs comes from future season payroll, ie, your player payroll for the first year of that contract. So if I sign a guy for 20M a year + a 10M bonus, his "cap figure" will be 30M out of player payroll for the first season of his contract.