Just talked to an escribano, yes, it's legal to do all the payment abroad. The issue is just if the owner doesn't have their account abroad declared, because that info goes into the escritura. In that case, the AFIP can easily find out that they have that account and they will probably have problems. So, it's unlikely that someone will say yes, because a lot of people don't declare their accounts abroad. In the case of an expat that's leaving the country, what do they care about the AFIP.
So Bill, want to sell me your apartment?