Your account has $20,000 in day trade buying power
At 10 a.m., you sell 100 shares of XYZ stock for $30,000. At 11 a.m., you buy 100 shares of ABC stock for $25,000. At 12 p.m., you sell 100 shares of ABC for $30,000.
You've incurred a day trade call because you exceeded your day trade buying power by $5,000. The sale of XYZ does not increase your day trade buying power, since it is based on start-of-the-day availability. At a purchase price of $250/share, the number of shares sold cannot exceed 80 without exceeding your $20,000 day trade buying power ($20,000 ÷ $250 = 80 shares).
Selling securities already held in your account increases your buy and hold buying power but does not affect your day trade buying power.