Question

Which 'sale' is Lady Adela talking about? Why does De Levis claim Dancy knew about the sale?

Answer 1

Lady Adela is talking about the sale of the Rosemary filly which was sold by De Levis to Kentman, the bookie, for an amount of nine hundred and seventy pounds. The filly was gifted by Dancy to De Levis.
De Levis was sure that Dancy knew about the sale because Goole told Dancy about the sale.