Probably this refers not to the restitution of Silesia, which he politely offered to her (though he had previously granted it on the Czar's request), but to Madgeburg and its environs west of the Elbe. On July 7th he said to Goltz, the Prussian negotiator, "I am sorry if the Queen took as positive assurances the phrases de politesse that one speaks to ladies" (Hardenberg's "Mems.," vol. iii., p. 512).
See the new facts published by Bailleu in the "Hohenzollern Jahrbuch" (1899). The "rose" story is not in any German source.
In his "Memoirs" (vol. i., pt. iii.) Talleyrand says that he repeated this story several times at the Tuileries, until Napoleon rebuked him for it.