It is not clear if Manafort was offering to trade information and cooperation or just looking for an agreement on a guilty plea in exchange for a recommendation for a more lenient sentencing. The latter would preserve a pardon strategy, but the latter would be a true game potential changer in adding a major cooperating witness.
If Manafort simply wanted a sentencing agreement in exchange for avoiding trial, the prosecutors could have concluded that they are likely to both win and get a high sentence in Washington. Without cooperation, there would be no advantage in trading away the trial of Mueller’s matinee defendant. Moreover, a new sentence may be enough to add to the desperation for Manafort in turning on Trump.
Manafort of course could simply plead guilty in Washington to seek leniency from the court. Such a cut down is not as significant as a deal to drop counts in exchange for a guilty plea. However, it can put a defendant at the bottom of the sentencing guideline.
The second trial is scheduled to begin on September 17th.