Dani, I understand what you are saying about the contract. I will need to check again, which might take a few days. But I can tell you that I have spoken to a contract co-pilot within the last three months who passed the command assessment but found that all aspects of his contract - days off, pay, base, etc. were to be changed when he was offered his command. (I will make no more comment on this as I do not want to all this individual to be identified).
All the evidence suggests that if you do not have a clear contract - or written agreement - IN ADVANCE that things will change afterwards. If you get yourself into debt to pay for the rating, it is a certainty that they will then feel you are trapped and will change your expected employment terms. Whether they do this through the agency or directly will not matter if they believe you have no choice but to accept!
On the other hand, I think it has been agreed elsewhere that the most reliable contracts at the moment are for those pilots coming through agencies. I am really saying that you need to be very careful as the usual rules of behaviour certainly cannot be assumed to apply in FR.