You know when Trump is lying...

...when his lips are moving. (Sorry, old Nixon joke.)

Slate has an article by Leon Neyfakh about Trump's latest move:

In a press conference in the Rose Garden on Friday, President Donald Trump (photo) took the extraordinary and surely very strategic step of accusing former FBI director James Comey of lying under oath at his Senate Intelligence Committee hearing. Trump denied unequivocally that he told Comey he hoped he would "let go" of the Michael Flynn investigation or asked Comey for a loyalty pledge, contrary to what Comey testified. Trump also said he would "one hundred percent" meet with Special Counsel Robert Mueller, who is overseeing the Russia investigation, and tell him the same thing under oath.
Trump appeared outside the Oval Office alongside visiting Romanian president Klaus Iohannis. After the two men gave speeches, there was a brief question and answer period, during which Trump was asked to directly respond to Comey's testimony in person for the first time. When a reporter asked “Are there tapes, sir?”, Trump replied that he would provide an answer to the question in "a short period of time", but added, "You're going to be very disappointed". It’s unclear if Trump meant to suggest reporters will be "disappointed" because there are no tapes, or because he believes the tapes would expose Comey to be a liar.
When asked about why he felt Comey’s incredibly damning testimony made him feel “vindicated”, as Trump had indicated on Twitter, given that it was his word against Comey's, the president replied, “No collusion. No obstruction. He's a leaker”, before moving on to a soliloquy about his unlikely electoral college victory over Hillary Clinton.
Trump’s exchange with ABC News White House correspondent Jonathan Karl, the part of the press conference that led the President to promise he would speak to Mueller under oath, is worth reading:

Karl: Mr. President, I want to get back to James Comey's testimony. You suggested he didn't tell the truth in everything he said. He did say under oath that you told him to let the Flynn— you said you hoped the Flynn investigation...
Trump: I didn't say that.
Karl: So he lied about that?
Trump: Well, I didn't say that. I mean, I will tell you, I didn't say that.
Karl: And did he ask you to pledge...
Trump: And there would be nothing wrong if I did say it, according to everybody that I’ve read today. But I did not say that.
Karl then asked Trump if it was true that he had asked Comey for a loyalty pledge, and Trump said it wasn't. (I'm not quoting this part of the exchange because Karl accidentally mangled the question and asked Trump if Comey had asked him for a loyalty pledge, prompting Trump to reply: "No, he did not". However, it's safe to say Trump knew what Karl was trying to ask him and responded accordingly, given that Trump's lawyer previously denied that Trump told Comey he needed and expected his loyalty.)
Karl: So, he said those things under oath. Would you be willing to speak under oath to give your version of these events?
Trump: A hundred percent.... I hardly know the man. I'm not going to say, 'I want you to pledge allegiance.' Who would do that? Who would ask a man to pledge allegiance under oath? I mean, think of it. I hardly know the man. It doesn't make sense. No, I didn't say that and I didn't say the other.
Karl: So if Robert Mueller wanted to speak with you about that...
Trump: I would be glad to tell him exactly what I just told you, Jon.
Karl: And you seem to be hinting that there are recordings of those conversations.
Trump: I'm not hinting anything. I'll tell you about it over a very short period of time.

Before anyone gets too excited, it's worth remembering that Trump has also made promises to release his taxes, and he has, so far, given no indication that he plans to actually do it. For the same reason, Trump's promise to speak to Mueller under oath should be treated with skepticism as well.

Rico says he's not gonna hold his breath... (And isn't 'a very short period of time' two weeks in Trump's world?)