For USD 400 to be withdrawn from the margin account, the trader's position must gain USD 400.

$`$4000F = (\20`,000 + \`400`) = 20,400$$F = \frac{20,400}{4,000} = 510 \text{ cents}$$

So, USD 400 can be withdrawn from the margin account if the price of wheat rises by 10 cents (510–500) or more.

Alternative (More Direct) Approach: The trader has a short position. Thus, they will gain if the price of wheat falls and lose if the price rises. The loss for each one-cent price increase is $0.01 \times 4,000 = `$40`$. Therefore, the price would need to rise by $400/40 = 10$cents for the trader to lose `$400and have$400` withdrawn from the margin account to bring it back up.

Additional explanation: In the case of a short futures position, the trader sells the futures contract with the expectation that the price of the underlying asset will decrease. If the price of the asset increases, the value of the short futures position will decrease, resulting in a loss for the trader. This loss will be deducted from the margin account, reducing the amount of available margin and potentially leading to a margin call if the margin account balance falls below the maintenance margin level.