The talented French winger was immediately linked with a move to Camp Nou following Neymar's world-record move to Paris Saint-Germain.
Dembele did everything he could to push the deal through and was subsequently suspended by Dortmund.
He was not the only man linked with Barca, with Liverpool's Philippe Coutinho also said to be wanted by the Catalan giants, but ultimately it was the France international who succeeded in getting his move.
Dembele will undergo a medical on Monday before completing his switch, the 20-year-old agreeing a five-year deal.
But while some were celebrating the move, Sammer, who worked as Bayern's director of sporting operations until 2016, is outraged that Dembele's behaviour has essentially been rewarded with exactly what he wanted.
"His strike brought him where he wanted," Sammer told Eurosport. "This is a catastrophe.
"In the interest of the cleanliness of football, such a behaviour should not be rewarded."