Can a temporary employment agency give me half of the salary applicable to the position within the organization?

Questioner

I work for an organization where I have just been promoted to a significantly higher position (at least 3 scales higher if I can believe Google), but I am employed here through a temporary employment agency that already paid me poorly (even for my previous position). In the conversation about my salary with the temporary employment agency, I indicated that I agreed to this in order to start somewhere and it was implied that we could discuss this again if I got a higher position. However, when I called them with the good news that I had managed to secure a new position (completely without their knowledge) within the organization where I was seconded, I was told that they did not do interim salary increases and that I would have to make do with a bonus. This would mean that I earn about !half! of what is stated on the internet that this position should yield at least. It is not possible to be employed by the organization. I am therefore curious, is this allowed and what are my rights and options? I do not have a relationship or non-competition clause. The contract of the secondment agency runs through an intermediary party that has the framework agreement with the organization.

Lawyer

No, this is not allowed! I would contact a specialized labor lawyer (member of a specialist association) as soon as possible. Often the first consultation (max. half an hour) is free.