Yes, you are right. v. Neumann was promoted Major as late as 30th August 1814.

I don't remember what my source was, I took these notes some years ago.

Counterchecking with Gumtau, Jäger und Schützen des Preußischen Heers, vol. 1, Berlin 1834, p. 341 f., he says the Prussians were marching "towards Montmirail". So mightbe this was the source of my error, as I didn't check the place and date. It is the combat at Janvilliers near Vauchamps, of course. Gumtau also confirms the strength of 230 men.

For the similarity of the reports, it seems either Otto is based on Gumtau, or both use the same source (maybe Neumanns's report preserved in the archives, if there was any).