House purchase financing from the handover date, what if the seller does not hand over?

No rent clause! As said, you have to conclude a real rental agreement for that. Penalty payment if he does not vacate on time. There can be enough reasons for this, e.g. you are building yourself and want to protect yourself in case the construction is delayed. Then just penalty payment.

Sorry, the OP never mentioned rent either. Due to the other posts, in which rent was always mentioned, I got carried away and also wrote rent instead of penalty payment. Still, we would first have to know the reason why the clause ended up in the purchase contract. If it is the delay of a new building you mentioned, I do not understand the OP’s concern that the seller could have to pay this penalty permanently. And even if the seller’s new building (if that is even the reason) should completely fall through, the OP should still have legal options to get his property. The background of the OP’s questions is still missing.

I must have been unclear. The buyer pays the rent for his apartment. He would have to continue paying it if the seller does not vacate the property to be purchased by the handover date.

In this case, the contract states that the seller has to pay the buyer a usage compensation of €700 per month.

But how long can the seller do this? Is there a legal limit? In doubt, the seller could be cunning and rent the house to third parties for €800 per month and still pay €700 per month himself. In a few years with inflation, it would still look better for the seller. Or he lets his child live there - lending him the property for free. The €700 per month does not really hurt the seller. The buyer can also endure that for 2-3 months. But the seller could play this game for 50 years.

Otherwise, the buyer has already paid the purchase price but still has to pay rent himself, which is even higher than the monthly compensation payment (not to mention that the buyer could even invest the purchase price with interest gains).

The exciting question is why there are only 700€/month stated in the contract. If there were an amount that made sense, all the discussions afterwards wouldn't even be necessary.

In this case, the contract states that the seller must pay the buyer a usage compensation of €700 per month.

Again: why include something like this in a notarized contract? It makes no sense. The assumption is that he will vacate the house, which no longer belongs to him, by the date. And if he does not, there is an eviction lawsuit.
One can (we did it this way too) postpone the handover date in the contract. And if he is not finished with his affairs by then, that is his problem. Either you come to an agreement or he has to stay temporarily elsewhere.

If in doubt, the seller could be cunning and rent out the house.

Again: No, he cannot rent out the house, because it no longer belongs to him.
Your construct is far-fetched and based on too much imagination. We remain in the legal reality and not in a well- or poorly-researched psychological thriller.

In this case, the contract states that the seller must then pay the buyer a usage compensation of €700 per month.

When is the latest handover date supposed to be? This must be agreed upon in the contract for the regulation to make sense.

But how long can the seller do this? Is there a legal limit?

No legal regulations – but according to the contract, there is a handover date.

In case of doubt, the seller could be tricky and then rent out the house.

Did you not read, understand, or are you ignoring my post? Landlord = Owner = not the seller.

“Due to his obligation to vacate and to pay the usage compensation, the seller subjects himself to immediate compulsory enforcement based on this deed.”
==> This is a standard text for such cases. If the seller resists, one obtains an enforceable copy of the purchase contract, goes to the bailiff, and after 14 days the bailiff enforces the eviction against the seller with the title.

: Please do not only take one sentence from the contract. Post the full passages. Then there will be better answers. But real estate. You will get rid of the seller very quickly, because you are the owner of the property and there is no rental agreement (and there can be no sublease agreement with a third party – and even if one was concluded, the property would then have been vacated by the seller and you would practically throw out the unlawful subtenant overnight; even here, in the worst case, shortly afterward with the bailiff).

No, he cannot rent out the house because it no longer belongs to him. Your construct is far-fetched and conjured up with too much imagination. We stick to the legal reality and not to a well or poorly researched psychological thriller.

The house no longer belongs to him one day after the handover date – even if a usage compensation is agreed upon? So you would simply recommend deleting the clause about usage compensation. If the seller then continues to use the property, an eviction lawsuit could be filed. With a usage compensation agreement, however, the seller could still use the property for many years if he pays the usage compensation amount.