Not a specialty of mine but as described by Henry Kissinger in A World Restored which I imagine might cover the diplomatic history better than some other sources it came down to the competing Prussian and Russian claims over Saxony and Poland which had been a sticking point in the negotiations. Several previous treaties promised to restore Prussia to its territorial extent in 1805, but did not specify that it would receive the same territories. So the question was where would Prussia regain the balance of territory, which deadlocked between Saxony and Poland. Metternich proposed an alternative solution:

Already in his memorandum of 10 December [1814], he had advanced a plan by which Prussia could be reconstituted to the scale of 1805 by acquiring territories in the Rhineland together with a portion of Saxony. This plan was now taken up by Castlereagh, when it became apparent that Prussia would not carry out her threat of war...

In his endeavour to achieve a final settlement, Castlereigh had to resist a new attempt by Prussia to move the King of Saxony to the left bank of the Rhine and an effort by Austria to save the Elbe fortress of Torgau for Saxony. But with the aid of the Tsar, he convinced Prussia that in the interest of the European equilibrium she would have to assume the defence of the Rhineland...

On February 11, a final agreement was reached. In Poland, Austria retained Galicia and the district of Tarnopol, while Cracow was constituted a free city. Prussia retained the district of Posen and the city of thorn which controlled the upper Vistula. The remainder of the Duchy of Warsaw...became the Kingdom of Poland under the Tsar of Russia. In Germany, Prussia obtained two-fifths of Saxony, Swedish Pomerania, much of the left bank of the Rhine, and the Duch of Westphalia....It was no accident that therefore that the contest over Poland fought in the name of abstract considerations of balance of power proved inconclusive, but that the dispute over Saxony involving the historical problem of Germany provided the key to the settlement.