Don't conflate Merck with Merck. There's two "Merck", one is Merck KGaA, which is a German company, and the other one is Merck & Co. Inc., which is the US company. These are independent entities today, they just share a common history by originally being a German company, of which the US company was splitted off during WW1.

The Oxford/Merck deal would have been with Merck KGaA, as far as I know. The US government would have had no means to apply export bans to a German-based company.