I have encountered this question before. This was my answer.
The United States pursued gradual engagement to the Allied nations of Europe by passing the Lend-Lease Bill.
Under this bill, the United States will provide Great Britain the necessary supplies and armaments that it needs to fight against Germany. Knowing that Great Britain's liquidity is questionable, the United States will not demand immediate payment and when the time comes that payment is demanded,they will accept other modes of payment beside cash.
In this scenario, the United States did not violate the Neutrality Act of 1939. They also ensured that the war fought in Europe will not reach their shores by ensuring that Great Britain has the capacity to fight and win the war against Germany. Thus, their assistance through lend-lease bill.