Buying land? - The standard land value in this area is 270 euros

How is it when you take out a low-interest loan? Does the delta to the current key interest rate count as a gift?

The person who receives the interest must pay tax on it. Possibly a monetary benefit for you (the Federal Court of Justice has decided), which must be taxed.

Whereas it is still significantly cheaper to tax 500 EUR as a non-cash benefit than to pay it to the bank in the form of market-standard interest.

For inheritance tax, one would have to pay 15% on 75,000 EUR and 20% on the remaining 105,000 EUR after deducting the 20,000 allowance, which amounts to 32,250 EUR.
That is a fairly manageable amount, considering that one receives a property worth 200,000 EUR in return.

He has now suggested to "somehow" pass it on. But I see major problems here like inheritance tax. Nephew to uncle is only 20,000 euros.

Do you start thinking about inheritance tax from a business perspective when you hear the word "inheritance tax"?
Assuming the property is valued at the standard land value. Then the value is €216,000. You subtract the €20,000 exemption and you tax €196,000 at 20%. That results in a tax burden of €39,200.
These almost €40k are then what the property costs you. And do you see a problem there?

Do you hear "inheritance tax" and think in commercial terms?
Assuming the property is valued at the standard local value. Then the value is €216,000. From that, €20,000 exemption is deducted and you pay tax on €196,000 at 20%. That results in a tax burden of €39,200.
These almost €40k are then what the property costs you. And do you see a problem there?

My uncle basically still wants to get money for it. After all, he acquired it back then for 270k. These €40k go to the state, he ends up empty-handed.
The discussion is pretty much over anyway because there are too many disagreements. On the one hand my wife, then the topic of sale/inheritance etc., and the unrest with "because of me you have everything here." His preferred solution would be:
Here is the property: build on it, when I die you get a "cheap" preemptive right. Although, as has been explained several times unfortunately it won’t be cheap, but just a deferred sale/inheritance.

My uncle basically still wants to have money for it.

That is then called a "sale" and not a donation.

However, the discussion is probably over because there are too many inconsistencies. On the one hand, my wife, then the topic of sale/inheritance, etc., and the unrest with "because of me you have everything here".

From the small insights into the situation, I think this is the right decision.

He would prefer the agreement:
Here is the property: build on it, when I die you get a "cheap" right of first refusal. Although, as already explained several times, unfortunately it will not be cheap, but only a postponed purchase/inheritance.

I have already written something about that: a really bad idea without a lot of legal guardrails...

Moreover, with regard to inheritance law, it is also not that simple.
First, relatives in the direct line are entitled to inherit and also to a compulsory portion (another nasty thing).
If none of them exist, only then can it be inherited elsewhere quite easily.
Which sounds doubly unfavorable here: first the property goes to someone else, from whom the property could then be "cheaply" bought. Doesn’t really sound ideal for one’s own house.
(And: yay, first inheritance tax for the heirs, then gift tax when sold below market value by the heir. And if it goes extremely badly, the municipality may exercise its right of first refusal. Okay, the latter is at least very unlikely.)