Certainly a treasurer knows everything. Just like the story of Shewakram, he knew everything about Tarachand and Lekhraj. He gave affidavit about the 10k rupees because he knew it, it was a fact.
Ushu Dadi was made a treasurer a bit later. Not from very beginning, officially she became a treasurer when the Yagya started expanding after they moved to Abu.
There is reference of this of BK Usha (Delhi) to whom Ishu Dadi would send a receipt of contribution:
“I started going to post office myself for posting my letters and sending monetary cooperation. I would, in turn, get a return slip from Ishu Dadi ji.”
In the beginning someone from the family of Lekhraj was in charge of keeping the key to the safe during the day, at night the key to the safe would go under the pillow of Lekhraj.