There was a very narrow window between the passage of the Neutrality Act of 1939, in November, which made possible the 'Cash and Carry' policy, and passage of the the Export Control Act of 1940, was passed in July. In theory, Japan could have qualified, but there were two important factors in plat. The first is that Japan didn't want all that much from the US anyways. As far as finished products like aircraft went, they were very happy with their own domestic industry. When it came to raw materials, some of which they did have interests in, there was strong extra-legal pressures on American businesses to avoid sales to Japan in any case.

Less than a month after passage of the Act, Roosevelt's administration was requesting companies that exported strategic resources such as aluminum and magnesium, to voluntarily cease exports to Japan.Even this wasn't that big a deal, since Japan had been pushing hard over the past few years to increase its self-sufficiency in aluminum production, which was key for aircraft manufacture of course. In 1936, they were only able to provide 40 percent of domestic need, but by the end of 1939 and the American de facto embargo, the loss of American imports barely registered.

So the short of it is that two things prevented Japan from 'taking advantage', the first simply being a lack of interest in much of what might have been on offer - why buy a F2A when you'll have the A6M rolling out soon - and the second being what Miller terms a 'moral embargo' on certain material and product that had obvious military application.

Miller, Edward S. Bankrupting the Enemy: The U.S. Financial Siege of Japan Before Pearl Harbor. United States: Naval Institute Press, 2012.