1. All oil sales contingent upon approval of the National Assembly. This is something that Maduro cannot abide, because he doesn't recognize the National Assembly. Getting their approval first would require Chavismo to recognize the NA. If the United States were to unilaterally establish an embargo, that would play into Maduro's "anti-Imperialism" hand. If he didn't get the approval of the NA, he only has Chavismo to blame.
2. All oil payments to Venezuela in food and medicine. Of which there is very little in Venezuela. How would he explain this to Goldman Sachs, the Chinese and the Russians? Maduro has some big bond payments coming due. I doubt his creditors will accept band aids and chicken breasts as payment. Again, this puts the onus upon Maduro.
Let it be clear that Maduro is merely the patsy. He isn't the power behind all of this. The Cubans (well over 25,000 well placed in the PSUV, the military, GNB, PNB and in Venezuela government over the last 18 years) would love nothing more than to add another pebble into Uncle Sam's sandals. Maduro would end up room temperature is he backed down.