Donald Trump, left, and Bernie Sanders. (AP)

Presumptive Republican presidential nominee Donald Trump on Friday suddenly backed away from suggesting he is open to debating Sen. Bernie Sanders before the conclusion of primary season.

In a statement, Trump took digs at Democratic presidential front-runner Hillary Clinton. He said he will wait until the nomination process is concluded and expects to eventually debate Clinton.

"Based on the fact that the Democratic nominating process is totally rigged and crooked, Hillary Clinton and Deborah Wasserman Schultz will not allow Bernie Sanders to win, and now that I am the presumptive Republican nominee, it seems inappropriate that I would debate the second-place finisher," Trump said in a statement.

Trump said Thursday that he would “love” to debate Sanders before the June 7 California primary for a good cause. He raised the possibility of using a debate with the Democratic underdog to raise millions of dollars for "women’s health issues."

Trump was still entertaining the idea of a possible debate with Sanders at a rally in Fresno on Friday morning. In that speech, Trump said that the television networks that host presidential debates are "very greedy" and make millions of dollars off his appearances, money that he would like to see given instead to charity.

But by Friday afternoon, Trump, who blamed television networks, had changed his mind.

"The networks want to make a killing on these events and are not proving to be too generous to charitable causes, in this case, women’s health issues. Therefore, as much as I want to debate Bernie Sanders — and it would be an easy payday — I will wait to debate the first-place finisher in the Democratic Party, probably crooked Hillary Clinton, or whoever it may be," he added.

Just before Trump issued his statement Friday, the Sanders campaign released a statement saying it had received two offers by broadcast television networks to host a debate.

“We are prepared to accept one of those offers and look forward to working with the Trump campaign to develop a time, place and format that is mutually agreeable. Given that the California primary is on June 7, it is imperative that this all comes together as soon as possible. We look forward to a substantive debate that will contrast the very different visions that Sen. Sanders and Mr. Trump have for the future of our country," said Sanders campaign manager Jeff Weaver.

Jenna Johnson contributed