Conte is adamant he made it clear to Costa during last season that he would be sold in the summer.
Costa was left out of Chelsea's pre-season tour of Asia and has been training away from the rest of the first-team squad since revealing the contents of a text message exchange with Conte at the end of last season.
Conte told Costa by text that he wasn't in his plans for the forthcoming season after the Italian grew tired of the volatile 28-year-old's attempts to engineer a transfer to his former club Atletico Madrid.
Costa's representative, Ricardo Cardoso, launched a scathing attack on Conte on Thursday, saying he had made it "impossible" for him to stay at Stamford Bridge because of the way the Blues manager had "dismissed" the Spain striker via text.
But Conte is adamant he made it clear to Costa during last season that he would be sold in the summer if a buyer could be found, so the text exchange was only reinforcing that message.
"For me it is very simple. I repeat what I said in the past; the player, his agent and the club knew very well the future of the player in the summer," Conte told reporters on Friday.
"For sure I don't understand why the lawyer gets involved. The situation is very clear. It is always the same. For me it's in the past."
Given the threat of legal intervention, a Chelsea spokesman, seated next to Conte at his press conference, said the club also felt compelled to respond.
"The premise the lawyer has put forward is just wrong," the spokesman said.
"The decision on Diego was made back in January. The player knew the decision, his agent knew the decision, clearly the lawyer has not been well informed.
"The lawyer's argument that Antonio as a coach has forced the player out by text message in June is just nonsense.
"We'll just have to see what the window brings."
Costa, valued at around £50 million ($65.39m) by Chelsea, would prefer a move back to Atletico, while AC Milan are also reported to be interested, but his former club are currently subject to a transfer embargo until January 2018.
As a result, Costa will remain exiled when Chelsea face Arsenal in the Community Shield on Sunday and is certain not to be involved when the Blues start their Premier League title defence against Burnley on August 12.