Don't sign anything your landlord is putting in front of you - especially not silently. The previous answers are right: broker fees are not legally part of operating costs and therefore cannot be passed on to tenants. That's pretty clear in case law. But here's the catch that people often miss: a lot of landlords just hope tenants won't question it and will simply pay along. It only works if you stay passive.
Do this concretely: demand from your landlord in writing (email is fine) that he give you the exact legal basis for this surcharge. In the same email, you lay out that broker fees do not belong to the operating costs that can be passed on, according to consistent case law. That's important because it documents that you're not just accepting it. Most landlords will quickly back down once it's clear that a tenant knows what he's talking about. If he stays stubborn, you can contact your tenant association or a lawyer - but honestly, most of them give up by then.
A practical tip: check whether the operating cost statement from last year already has errors and whether other illegal items have snuck in there. A lot of landlords throw multiple things in, and some of them might already be problematic individually. Then you can address all the points at once and have more leverage.