How do brokers negotiate the purchase price?

It doesn't matter whether I brought in the prospect or the broker did.

It depends on what kind of contract you have with the broker. There are different possibilities.

If the broker contract runs for 1 year & the prospect buys after 1.5 years - does the broker still receive a commission?

Difficult legal issue. It depends again on the specific circumstances of the individual case.
However, it hardly ever occurs in practice (why should the prospect buy only after a waiting year?).

It may be that with high amounts, the broker fees are to be saved. At 300,000€, 6% is 18,000€. Then one waits another year or half a year until the broker contract has expired.

At €300,000, 6% is €18,000. Then you wait another year or half a year until the broker contract expires.

Then the broker earns their commission.
That's what I mean by "it depends." If the buyer purchases under completely different conditions, the broker is out.
In practice, however, no one waits a year. If I have that much time (if I sell, I have something new— and don’t leave the old place empty for another year, ideally still financed), I sell calmly without a broker.

Your idea a) doesn’t work if you want to cheat the broker b) is unrealistic.

Which version a)? You can easily leave the place or the undeveloped land for another year. For €18,000! That is very practical.

This is how it appears to me: You have signed a contract and now you no longer like it and are looking for ways to circumvent the contract. This is how it seems to me: You are not willing to stand by your decision, the resulting consequences, and your word when you have a financial advantage from it.

I don't have a contract at all. I was just wondering because I was riding my bike and there was a real estate agent's advertisement sign on a property. After a few days, it was no longer there. That made me wonder how such contracts are regulated - how a real estate agent protects themselves. Otherwise, my approach would probably be common practice. Then you would just always let the contract expire. I don't want to become a real estate agent either. I just find it interesting.