“€*** per annum paid monthly in arrears by credit transfer, plus a quarterly bonus of €***.
This has always been guaranteed.
How long have you been receiving this for?
If you have been receiving it for a few years then custom and practice would also support what looks like a pretty clear contractual entitlement.
Have you received salary increases since you started?
What is the bonus as a percentage of your salary?
If they agree to pay you the bonus of €1,000 a quarter, can they, in effect, claw it back through not increasing your salary?
But if it's 25% of your salary, then that would be different.
I would initially write a nice letter and not be quoting the law, at this stage.
"Dear Employer
I took the job on the understanding that the bonus was guaranteed and indeed it has been paid for the past 10 years so my understanding was correct. I always regarded it as integral to the basic pay structure.
It was critical to my decision to take the job. I could have got a higher basic elsewhere but with no bonus.
I am glad I did because I enjoy my job and working for the company.
I welcome the introduction of a target based bonus system, but before it will be introduced, it will be necessary to increase my basic pay by the amount of the guaranteed bonus. "