Trump's "official" stance is clearly a wait and see how things are going but he did indicate that he is no longer bound to his debate agreements. He posted on Truth Social:
My debate with Crooked Joe Biden, the Worst President in the history of the United States, was slated to be broadcast on Fake News ABC, the home of George Slopadopolus, sometime in September. Now that Joe has, not surprisingly, quit the race, I think the Debate, with whomever the Radical Left Democrats choose, should be held on FoxNews, rather than very biased ABC. Thank you! DJT
In other words he is not committing to and not ruling out the possibility of debating whoever the Democratic nominee will be.
https://deadline.com/2024/07/trump-debate-kamala-harris-abc-news-1236017141/