Continuing to export food in a famine wouldn't have been unheard of, though it was rarely seen as particularly acceptable practice. The Irish Potato Famine saw food continue to be exported from Ireland. Mike Davis discusses similar events in the late 19th and early 20th century in Late Victorian Holocausts: El Niño Famines and the Making of the Third World (confession - have read this one myself). Even after the Great Famines in the USSR there was the Bengal Famine of 1943. Much of this times into a broader literature on famine, which sees these rarely occuring due to genuine lack of food, but from inequalities.

Why did exports continue in the USSR during the famine?

• Export credits were used for inputs for industrialisation (e.g. machinery), which was a major focus of the party-state.

• Initially there was a denial of the problem - in correspondence Stalin appears to believe failures of grain collection were a result of a lack of political will, not a collapse of agricultural production.

• I suspect the prospect of continued wastage (e.g. rot and so on) if grain for export was shipped back would have been an issue. Soviet transport and storage infrastructure was terrible and they already 'wasted' a lot of grain just getting it to the cities.

If you're looking to read something a bit different and/or with a counter-viewpoint to Conquest I would suggest Davies and Wheatcroft's "The Years of Hunger: Soviet Agriculture 1931–1933". Basically where Conquest sees the famine as deliberately inflicted, Wheatcroft and Davies argued it was the result of bad policy decisions. On the specofic topic of aid to famine effected areas they've got a breakdown of what was released and when.