It is believed that the 22-year-old has it written into his contract that he can leave his Portuguese employers if another club offer £42.5m.
Chelsea have been linked with a move for centre-back but are not said to be prepared to part with that sort of money. However, Da Costa has told the Blues and other sides that his club have no plans to drop the asking price.
"This footballer only leaves our club if someone pays the release clause," he told A Bola.
Mangala joined Porto from Standard Liege in 2011 for a reported £5m.