Property tax claim from the property seller

You have very questionable principles.

That is putting it mildly. Other terms come to my mind...

I knew it up to now that the tax liability always arises for the owner on January 1st. This would also make the seller liable for taxes. Of course, this can be arranged differently in the notarized contract, but if nothing specific is stated there, in my opinion, there would be no claim for compensation.

I had understood until now that the tax liability always arises on 1.1. with the owner. This would also make the seller taxable. Of course, this can be regulated differently in the notarial contract, but if nothing concrete is stated there, in my opinion there would be no claim for compensation.

The standard phrase "Rights and obligations transfer to the buyer from dd.mm.yyyy with payment of the purchase price" also includes public burdens. However, since property tax is only assessed and collected once at the beginning of the year for the entire calendar year, compensation to the seller is due. Completely normal.

I also find that completely normal and we have handled it that way with our sellers. Why should the seller continue to bear the costs for a property that does not belong to him.

It is true that this would be normal and usual. The OP probably wouldn't have asked if there was a regulation in his case. We bought our property from the municipality in December and paid for it in January. We have only been paying property tax since the year after. Likewise, we sold a part later, and there too we paid the land charge for both parts until the end of the year.