Neither Roosevelt nor the members of his Soviet Protocol Committee (which effectively determined lend-lease policy for the Soviets) were prepared to consider any reduction in lend-lease shipments to the Soviet Union following the Battle of Kursk, or even in mid 1944.
Roosevelt, through his closest advisor Harry Hopkins, kept close control of lend-lease to the Soviet Union, seeing it as vital in gaining Soviet trust in the long-term. This was despite there being people in both the US government and in the military who argued for a reduction in lend-lease to the Soviets from at least mid 1943.
A memo dated 4 February 1944 from Harry Hopkins, Chairman of the President’s Soviet Protocol Committee, to Averell Harriman, U. S. Ambassador in Moscow, clearly states the administration's policy:
...since no one can now determine when the war will be over, it seems
preferable that there should be no interruption in the procurement of
supplies for the USSR war program...
The arguments used to justify a reduction were mostly about the misuse of supplies. There was also advice against providing military equipment which strengthened the Red Army and warnings that the Stalin was taking advantage of American generosity. These arguments, though, did not come from those who dictated policy.
Despite these concerns, there was no reduction in supplies until Truman became president.
In Lend-Lease in Early Post-war Soviet-American Relations, K.V.Minkova notes that
even in the early stages of Lend-Lease, some officials from Roosevelt’s team tried to persuade him to limit
the U.S. aid to Moscow (at least by those positions that directly
strengthened the military power of the U.S.S.R. — e.g. by aircraft) or
to provide this assistance on a reciprocal basis — i.e. in exchange
for gold, strategic metals, etc. Admiral W. Standley, who preceded
W. Averell Harriman as U. S. Ambassador to the U.S.S.R., kept warning
Roosevelt that the Soviet leadership was taking advantage of his
Standley's concerns were dismissed by Hopkins, and those in charge of policy making evidently felt the Admiral was damaging US - Soviet relations, as witnessed by this memo The Executive of the President’s Soviet Protocol Committee [J. D. Burns] to the President’s Special Assistant [Hopkins], dated August 10th 1943:
We now have a number of United States representatives in contact with
Russian representatives who do not trust Russia and who do not follow
a national policy of the “good neighbor and sincere friend” to Russia.
They obviously do not develop mutual trust and friendliness. These
should either be replaced or they should be required to pledge loyal
support to the above policy.
Unsurprisingly, Standley was recalled from Moscow on Sept. 18th (though this had been on the cards since at least May). Burns also states the importance of the Soviet contribution in tying up German divisions and that she was critical in the effort to defeat Nazi Germany:
In War II Russia occupies a dominant position and is the decisive
factor looking toward the defeat of the Axis in Europe. While in
Sicily the forces of Great Britain and the United States are being
opposed by 2 German divisions, the Russian front is receiving
attention of approximately 200 German divisions. Whenever the Allies
open a second front on the continent, it will be decidedly a secondary
front to that of Russia; theirs will continue to be the main effort.
Without Russia in the war, the Axis cannot be defeated in Europe,...
Despite this, concerns over the way the Soviets were using the supplies the US was sending through lend-lease were being raised by at least mid 1943. Citing evidence which includes the Minutes of the Executive Staff Committee, Office of Lend-Lease Administration, July 13, 1943, George C. Herring, Jr in Lend-Lease to Russia and the Origins of the Cold War, 1944-1945 in the The Journal of American History (1969) states that the Soviets
...had requested vast quantities of industrial equipment which could not
be made operational before the end of the war and which was obviously
intended for postwar reconstruction.
Edward R. Stettinius, Jr, then Under Secretary of State, expressed similar concerns in a memo to Dean Acheson dated December 27, 1943. Disquiet over the misuse of supplies was particularly evident in the latter half of 1944. Citing W. Averell Harriman, then US ambassador to the Soviet Union, Herring also notes that later, in 1944, the Soviets
...were giving or selling to other countries American supplies or
items similar to those received under lend-lease in order to boost
their own political influence in Eastern Europe and the Middle East.
More pointedly, and edging closer to concerns over the Soviet advance into Eastern Europe, Major General John R. Deane, then chief of the United States Military Mission in Moscow, wrote a letter to Roosevelt's chief of staff, George Marshall on 2 December 1944:
'Everyone will agree on the importance of collaboration with Russia - now and in the
future [but] it won't be worth a hoot, however, unless it is based on
mutual respect and made to work both ways . . . when the Red Army was
back on its heels, it was right for us to give them all possible
assistance with no question asked ... the situation has changed, but
our policy has not.' Clearly irked, Deane went on: 'Some will say that
the Red Army has won the war for us. I can swallow all of this but the
last two words.'
Source: Diane S. Clemens, Averell Harriman, John Deane, the Joint Chiefs of Staff, and the 'Reversal of Co-operation' with the Soviet Union in April 1945 (The International History Review, May 1992)
Nonetheless, although not unconcerned about these 'abuses' in the way supplies were being used, the policy makers did not reduce aid:
...lend-lease to Russia was given a unique status....As the military position of the Allies improved
after mid-1944 and American troops took on a greater burden of the
fighting, lend-lease to most nations was sharply reduced.
None of these limitations applied to Russian lend-lease....Difficulties in transporting supplies to Russia imposed severe limitations
on the lend-lease program until 1943, but as the shipping crisis eased, protocol commitments
steadily increased and were often exceeded.
This is fully consistent with the views expressed by the Soviet Protocol Committee in mid 1943. Not until Truman became president did the policy significantly change.