Also keep in mind that boleto is not mandatory. So whether there will be a boleto or not is part of the negotiation. Is very likely that if they want a boleto, they have to sell this property to buy another one, so they need the money (as downpayment for example). So a boleto implies you have to pay more money early on. A boleto doesn't offer much advantage to the buyer, unless it's signed in front of a escribano.
Normally seller chooses the location of the money transfer and the buyer chooses the escribano. Don't be shy to negociate and ask for your own conditions.