Congress never signs off on Lend-Lease or any assistance for the Soviets. But they do immediately sign off on expansion of the Army to 1.5 million men and landing craft for a potential AEF should America get dragged into the war. The Navy is also expanded and that includes the Marine Corps.
You are slightly off on event dates here. Congress passed the 'War Powers Act' shortly after France colapsed in June 1940. That legislation authorized the complete call up of all Army and Navy Reservists, Activation of the National Guard into Federal service, taking in enough volunteers & draftees to bring the US Army to 1.4+ million men within a year (by June 1941), and purchasing equipment for a army & navy of over two million men. that was the inital mobilization. The Army and Navy Chiefs of Staff were directed to prepare mobilization plans for expanding the US military to over four million men by the end of 1942.
(If you mean the US starts mobilization in 1939 then your remarks make more sense.)
This was all authorized in the summer of 1940. The Lend Lease program was not started until a bit later in the autum of 1940, after the British government made it clear they would be bankrupt by 1942 were cash payments for material to continue. From 1938 to late 1940 Britain & France paid cash, or used gold deposited in North American banks as credit security for purchases from the US. The French were fairly well set for 'cash' with fair sized gold reserve and a adaquate flow of valuable commodities from their colonies. That had allowed them to contract enormous orders in the US. When France fell there had been firm orders for nearly 5000 aircraft placed with US factories (600 already built), and projected orders for another 10,000 in 1941-42. Britain was making similar aircraft orders with US industry. Both the French & British were contracting catch up maintiance & up grades in US ship yards, and there were enormous raw material orders from the US. All that was without Lend Lease.
However, they do not sign off on immediately aiding France, preferring to let France stabilize the lines should they happen.
Then of course France falls shockingly, scaring Congress into holding on. Battle of Britain however convinces them that Germany is not invincible but Congress is still wary.
So at this time, the Army is at 1.5 million and FDR ask for Lend-Lease to Britain and the Spviets. It is only approved for Britain and slightly limited.
This makes sense in the historical context. The early mobilization plans for the US Army provided for a realtively small overseas expeditionary force. The prewar cadre of active & reserve service NCOs and officers only allowed for a 1.5 million man force. To expand the Army to 4 - 6 millions required at least 700,000 to 800,000 men remain in the US as a training establishment for the three million plus raw recruits that would need to be trained.
Historically the US had approx 20 divisions & about 8 corps HQ with support units ready for expeditionary service by the middle of 1942. There were about fifteen other divisions at full strength in mid 1942, but they had to stay in the US to support the creation of the next batch of 40 divisions scheduled to be ready in mid 1943.
The main obstacle to US mobilization was the lack of a detailed industrial plan. The US Army Chief of Staff in 1919, Peyton Marsh had proposed such a plan be drawn up, but Congress never budgeted the funds for the staff this would require, despite appeals from the next CoS Pershing, Sommervell, MacArthur, Craig, and Marshall. When Roosevelt directed his Chiefs of Staff to present such a plan in 1940 they had little more than a few hundred pages of general requirements and estimates. It took over two years to organize and execute a practical industrial program for arming the US and its Allies. Had US industry not received a initial capitol investment from Britain and France in 1938, 39, & 40 from their arms & raw material orders, the US industrial mobilization could easily have dragged out 6-9 months longer.
There are a variety of books on the subject of US industrial mobilzation. A recent one I'd recommend is 'Freedoms Forge'. It focuses on the role of business leaders like Henry Kaiser, Knudsen, or Edsel Ford in converting and build factories.