Barr said publicly that he wouldn't object to Mueller testifying, and I expect that Mueller would want to show up and clear up any misunderstandings when he is called, especially with Barr calling his letter "snitty".
It all comes down to Mueller addressing Barr stating this though: “If [Mueller] had found enough evidence for an obstruction charge, I think he would state it,” which directly contradicts the rationale Mueller laid out in his report. Mueller indicated in a legal analysis of obstruction of justice that “fairness” dictated he not reach a formal judgment on whether the president obstructed justice — regardless of the evidence, and that “The conclusion that Congress may apply the obstruction laws to the President’s corrupt exercise of the powers of office accords with our constitutional system of checks and balances and the principle that no person is above the law,” Mueller’s team wrote.
