Key legal question
Whether the 1999 agreement was valid despite lack of authentic form.
Extracted holding
The agreement was void because the land sale formed a global mixed contract whose ancillary reciprocal prestations also had to be included in an authentic deed.
Extracted reasoning
Under Art. 216 CO, authentic form must cover all objectively essential and subjectively essential elements affecting the exchange. Here the sale was inseparable from other promised prestations, so the absence of authentic form invalidated the whole agreement.