Can a client simply impose fines and offset them against your invoice?

Questioner

I work as a self-employed person and provide courier services, I worked for the same client for a while, after working there for a while I decided to stop working for him and start working for another client, but now the old client is fining me 1500€ because I don't want to work for him anymore. He wants to keep this from me. Invoice that I sent him. Is he within his rights to do this. There is no talk of contracts that have been signed in which this is stated, he just makes this up. That is why my question is, is he allowed to do this and am I within my rights to refuse this fine?

Lawyer

If no penalty or something similar has been agreed, the client cannot rely on it. It is possible that his argument is that a reasonable notice period must be observed and by not doing so, the client suffers damage.