Why should we believe Mueller when he says Manafort is lying?

David Oscar Marcus:

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... One such issue is demonstrated in the Paul Manafort case, where the prosecution team just filed a status report with the court explaining that they have concluded that Manafort is not fulfilling his end of the plea agreement because, they say, he has lied to them during interviews (or as they are called in the system, debriefings). Manafort has said he has answered all of their questions truthfully. This may or may not be true.

But who decides? Strangely, the Mueller team is the decisionmaker in whether Manafort is telling the truth. In the Manafort plea, just as with all other cooperation deals in the federal system, the government gets to decide unilaterally whether to ask for a sentencing reduction based in part on whether they believe Manafort is telling the truth. Manafort cannot himself file a sentencing reduction motion under the sentencing guidelines, and neither can the judge. The government and only the government is charged with evaluating whether Manafort has provided “substantial assistance in the form of truthful information.”

Defendants quickly learn what this really means: Tell the government lawyers what they believe the truth to be or get burned at the stake.

This is what is expected to qualify as providing substantial assistance. The “truthful information” must be the version of events that supports the government’s storyline.

So long as the testimony supports the government’s case, they are happy. Even when cooperating witnesses who parrot government narratives are exposed as liars under oath at trial, prosecutors still file motions to reduce their sentences. In one recent federal trial in Orlando, a judge called out a cooperating witness and dismissed the case against the defendant. The prosecution team still reduced that witness’s sentence because in its view, he told the truth (i.e., he said what the prosecution wanted him to say).
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Mueller has already brought questionable cases against Gen. Flynn and George Papadopoulos for allegedly lying to investigators.  This was after the people Flynn actually spoke to said it was not lying and in Papadopoulos's case it was after being set up by someone working for the FBI. 

The purpose of these indictments was to extort testimony as was the indictment of Manafort on charges unrelated to the Trump campaign.  It looks like he is trying to use the same scheme to leverage testimony out of Manafort more to his liking.

One of Mueller's targets refused a plea bargain on a charge to lying to the FBI because the Mueller team allegedly wanted him to lie and he refused.  That could be what Manafort is thinking too.