[Verginius Rufus]

I think it would have been a little later. In late 1944 the British government pressured King Peter into calling for all of his subjects to support Tito’s resistance (rather than the Chetniks), and some members of the government-in-exile joined the Tito-led government in Belgrade after the Partisans took Belgrade with the help of the Red Army. Those ministers quickly learned they had no power at all. I think it was in late 1945 that the monarchy was officially abolished so the confiscation of the king’s property probably took place after that.

[annalex]

Since the examples of the Chetniks collaborating with both the Germans and the Axis were numerous, it was easy to accuse His Majesty of anything all throughout the war: as any nationalist movement would be, they had the interests of Greater Serbia and not any other party at heart. The word "proclaim" suggests that Tito had some authority at the time, this is why I think it would have been after he was "appointed Prime Minister of a joint government". Yes, any time after that, and certainly before the confiscation.

This source, by the way, makes a point that the "abolition" of the monarchy was illegal, by a mere decree, like everything else the communist gangs do.