A dealer buys 10,000 shares of ABC common at $20 for its inventory. One week later

$22

Explanation:

According to the FINRA 5% Policy the mark-down if the customer sell must always be calculated from the inside bid price which is here $22 and if the customer is a buyer then the mark-up must be calculated using the inside ask price which is here $23. As the customer here is seller, hence the inside bid price $22 was appropriate here according to the FINRA 5% policy.

FINRA 5% policy says that the broker can not charge commissions, or markups or markdowns which is more than 5% on standard trades.

So the commission of $2 ($23-$21) is not allowed under this rule hence the appropriate price for the stock must be $22 not $21.