The woman who has said that she does not wish to be identified has said that she will be keeping the note as na investment for her grandchildren,
The banknote was one of four micro-engraver Graham Short, 70, etched with tiny 5mm portraits of Jane Austen.
He said that he had spent it in the Square Cafe in Blackwood, South Wales, last Friday.
Staff in the eaterie unwittingly gave it away in change.
Owner Josh Ince, 33, was left checking all his fivers when it emerged the priceless banknote had been sitting in his till.
The special note was spent on a sausage and egg butty by Mr Short.
Mr Ince said: “We didn’t even know about it until Monday, one of our staff heard it on the news.
“I told everyone to check all the fivers but it wasn’t there.
“No one noticed at the time, but when we saw a picture of him online, some of the staff recognised him.
“He came and had a sausage and egg sandwich.”
The fiver includes a tiny portrait of novelist Jane Austen measuring just 5mm.