You can just subtract the two values:
SELECT (SELECT COUNT(ID)
FROM Used
WHERE ID = 54
AND QTY = 1.875
AND DateReceived = '2014-03-27 00:00:00'
AND VendorID = 12400
AND WithDrawn = 0) -
(SELECT COUNT(ID)
FROM Used
WHERE ID = 54
AND QTY = 1.875
AND DateReceived = '2014-03-27 00:00:00'
AND VendorID = 12400
AND WithDrawn = 1);
Alternatively, construct a value of +1 or -1 for each record, and take the sum over that:
SELECT SUM(CASE WithDrawn WHEN 0 THEN 1 ELSE -1 END)
FROM Used
WHERE ID = 54
AND QTY = 1.875
AND DateReceived = '2014-03-27 00:00:00'
AND VendorID = 12400;