PsicoKehec -
Where did you read about this pledge from Tito to Stalin. The most similar thing I ever read was in Gerhard Weinberg's "A World At Arms", but he described it as Tito's own plan to hang back while the Germans defeated the Western Allies, and it was not discussed with reference to Stalin in that instance.
I discussed this with Doug Muir in soc.history.what-if at
https://groups.google.com/d/msg/soc.history.what-if/nNcBC-mfAMk/anYLQf7jlsoJ
Tito did urge the Germans to give him a free hand vis-à-vis Mihailovic, and said that if they did that, he would cooperate with them against the Wallies. To my question of whether he meant that or was just "bulljiving" (to use an expression of a late Chicago alderman) the Germans, Doug replied:
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Bulljiving, I think.
Oh, Tito was very worried about the idea of an Allied landing, no
question. He repeatedly told the British that it was a bad idea, and
at one point he even told Stalin that the Partisans would resist it.
However, this seems to have been bluff. Djilas and others say that he
was resigned to the possibility, and convinced that Communism would
prevail anyhow. After all, he had the men with the guns. And by the
time an Allied invasion was seriously in the air (summer '43 onwards),
the Royalist government had lost almost all credibility in Yugoslavia
outside of Serbia. So Tito would have been able to spin them as a
Serb nationalist regime "arriving in the baggage" of the Allies.
Note that young Prince Petar was an unknown quantity outside of
Serbia, and not particularly popular. The fact that he'd assumed the
throne by means of a coup by hardline Serb nationalists did not make
him particularly attractive to non-Serbs.