I cannot set the item's Sales Unit when I register an order via SOAP

Hi,
I cannot set the item's Sales Unit when I register an order via SOAP.
The POST request sends something like:
...
SAU: "UN",
QTY: "",
SAUSTUCOE: "1",
STU: "UN",
...
But the order resets to the standard "PCT" sales unit.
can anybody help me?