Cost coverage for fencing that is not desired

I have said that several times already, but he doesn't care. He wants to start now.

What reason could he have for that? Is there a pet that could run away? Does he need the sense of security when the kids are playing outside? If you don't know, find out. If there is a good reason, it will be easier for you to agree. At the moment, it's not clear to me how many stubborn people are involved.

300 euros for a fence should still be doable.

But if the neighbor wants a high-quality fence from the landscaping company, that can easily be the price per running meter. So at 12 meters length, maybe even 4k, and the neighbor would like 2k. I would also go on the barricades there – or give him the 300€ (that is what I would spend on a chain-link fence) and then enjoy the neighbor’s fence.

The neighbor probably already has a precise plan in mind for how he wants to design his garden this year. If in some places the planting is supposed to border directly on the fence, it also makes sense to set this up first and then start with the rest. However, of course, I can understand that the enclosure is not yet an issue for you. If I were you, I would arrange a joint appointment, wait for the offer, and then decide. In parallel, I would find out roughly what a chain-link fence would cost, if you only want to take care of that part.

Even a very expensive fence would be annoying, but I would accept it after a conversation didn’t work out. The OP, however, does not show their constructive side here. Any advice that preserves the relationship with the neighbor is the best. And sometimes that just costs money. The money that would sustainably improve the OP’s life. A spoiled neighborly relationship, on the other hand, can have a really bad influence. Sometimes you just don’t have the best seat in life. As long as it’s limited to a garden fence, you can only congratulate the person.

Any advice that spares the relationship with the neighbor is the best.

Yes. My advice, however, is that with a neighbor who doesn’t even ask when it comes to joint decisions, you should definitely not just be an automatic yes-man.
What kind of relationship will that be with the neighbor who doesn’t care about my own opinion and does whatever he wants? A "He does what he wants" and "I just say yes to keep the peace"? No. Definitely not.

I am also someone who wants peace, but this is not how it works at all.

I stick to my point: I would kindly tell the neighbor that I will of course bear half the costs of a chain-link fence, as the law provides. And he is of course free to install the 1.2m palisade fence and pay the excess price himself. Unless they agree on something that both equally want, then the costs will be shared brotherly.

So your neighbor has alternatives: immediately have his fence and possibly pay more than half. Wait and pay exactly half of whatever it is.

I would not have the discussion that "we are not ready yet" with someone who already lives there. Priorities are understandably quite different there.