When you have a promise to sell or a sales agreement and well you can retract. To do so you must send by registered letter with an acknowledgment of receipt from the day after signing within 10 days. This delay is essential because it allows you not to have to justify yourself and also avoid financial penalties. 

Here is the letter that will be requested: In red these are the elements that are personal to you and are to be completed. The rest is a legal form that will be requested.

In addition, you can also ask your notary for help in writing this letter.