You have a very mistaken view of Soviet trade, if you think "we promise not to develop nuclear weapons, if you give us grain!" It's wishful thinking, akin to a leftist claims that the USA gets all its manufactured goods by enslaving developing countries.

The Soviet Union imported large amounts of grain due to a deeply broken agricultural system, which I believe we're in full agreement on. But this wasn't in exchange for merely making unverifiable promises to the West (why would the United States have agreed to that, even?). Much trade was with other socialist countries, for one. And although Western countries received few Soviet manufactured goods, they, then as now, imported large quantities of Soviet commodities, particularly oil and natural gas.

Indeed, its reliance on oil for hard cash is what led in part to its downfall, as oil prices collapsed in the 80s, putting significant strains on the system as the pie got much smaller while more and more resources were needed to satisfy rising consumer demands and ill-conceived land wars in Asia.