Buying property in Bulgaria – deposit procedure

Hi everyone, I’d really appreciate your advice.

We’ve found a property we like in Bulgaria. Since we’re currently abroad, my boyfriend’s brother viewed it on our behalf. We agreed on a price with the owner, and to secure the property we suggested sending a deposit (1,000 BGN).

Today, my boyfriend spoke with a notary who advised that we should sign a preliminary agreement and asked for an amount (400lev). Is this the correct procedure? Also, we’re unsure who is supposed to hold the deposit until the final deed is signed (which we plan to do in about a week and a half). The notary said the owner usually keeps it and it is then deducted from the final purchase price.

For context: my boyfriend is Bulgarian, but we just want to make sure everything is done properly. Has anyone gone through a similar process?

Thanks so much for your input!