I really think the question is being answered without enough information.
Big question is, what happens if buyer fails to get mortgage and the money? My expectation is that he just forfeits the deposit, you keep the house, and that's the end of it.
If the seller is concerned, he should double-check this with another independent, reputable lawyer in Budapest, not just an Irish online public forum. The laws are very different in every country, and local expertise is needed. We should not be giving 'Irish' legal advice for a Budapest situation.