Preliminary agreement (umowa przedwstępna)
An agreement in which the buyer and the seller undertake to conclude a sale agreement in the future on agreed terms. Among other things, it sets the price, the date and the method of payment.
It can be made in ordinary written form or in the form of a notarial deed. The notarial form gives stronger protection: if the other party backs out, you can go to court to demand that the sale agreement be concluded, not just claim damages.