In the case of a unilateral promise to sell, only the seller is obliged to sell her/his property; the buyer has a purchase option within the agreed time period. Should the latter not exercise her/his option, s/he will lose the security deposit (known as the immobilisation compensation) that has been paid, unless one of the conditions precedent provided for the sale to take place has not been fulfilled. If the seller refuses to sell, the courts can compel her/him to do so.
In the case of a bilateral undertaking to sell (agreement of sale or pre-contract), both parties are obliged to fulfil their promises: the seller is obliged to sell and the buyer is obliged to buy.
However, in both cases, these preliminary contracts are subject to conditions precedent (verification of title to property, enforcement of the right of first refusal or of preference, the granting of a loan, etc.), which are preconditions to the sale going through. If any of these conditions are not fulfilled, the final sale will not be concluded.
The preliminary contract also specifies the time span within which the sale has to take place. As a rule, it is within 2-3 months.