It seems like a strange coincidence that Sai Baba mentioned tea with sugar right after Cholkar fulfilled his vow. Is there any actual proof in the text that Baba knew about Cholkar's secret sacrifice all along?

The text strongly suggests this was not a coincidence but a demonstration of Sai Baba's omniscience. As detailed in Chapter 15, Baba did not normally drink tea, so his sudden mention of it was unusual. More importantly, Baba explicitly told Jog to give Cholkar tea "well-filled with sugar." After Cholkar's amazed reaction, the narrative explains that Baba gave a "full hint," saying, "Your vow of sugar has been accepted, And your rule of renunciation is also complete, O Cholkar." Baba further states, "The state of your mind at the time of the vow, and the penance of the long delay— Though you kept these secret, all of it is known to me." This direct statement from Baba confirms he was fully aware of the secret vow and sacrifice.