Since the total sales value remained the same in spite of entering with their digits interchanged, both its digits must've been the same.
But the inventory reduced by 54.
Suppose the number to be entered was xy = 10x + y which was mistakenly entered to be yx = 10y + x
xy - yx = 54
10x + y - (10y + x) = 54
9x - 9y = 54
$\therefore$ x - y = 6 ...
From the given options, only C satisfies the above equation.
C is the answer.