The United States calling for neutrality began their policy of lend-lease, where they give military weapons to the allies in exchange for what?

Essentially, the answer to this is "nothing."  The policy
was called "Lend-Lease" mostly to make it sound good, not because there was actually any
serious exchange going on.

Lend-Lease was really a program
of straight lending.  The US was lending money to the Allied countries by giving them
war materiel without requiring them to pay for it up front.  If the Allies won the war,
they would repay the value of the materials that they had been
given.

The act covered itself by saying, for example, that
the US would not be required to pay the rent on some bases that it had on foreign soil,
but the value of this rent was really much lower than the value of the goods sent to the
Allied powers.  So, as the link below says

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Although the terms were complicated, Lend-Lease
really meant that Britain could buy American goods—including weapons—on credit and pay
back the money after the war.