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
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.