Updating the quantity on a Sales Order Item does not update the item's gross amount!

I am trying to update the quantity on a Sales Order Line Item. The quantity value changes, but the line total amount is not updated!

The update() call returns a warning indicating that the line's total amount is incorrect!

If I try to update the line's total amount, the new amount is ignored!

proxy.preferences = new Preferences();
proxy.preferences.useConditionalDefaultsOnAdd = true;
proxy.preferences.useConditionalDefaultsOnAddSpecified = true;
proxy.preferences.useConditionalDefaultsOnUpdate = true;
proxy.preferences.useConditionalDefaultsOnUpdateSpecified = true;

RecordRef request = new RecordRef();

request.internalId = "2302";
request.type = RecordType.salesOrder;
request.typeSpecified = true;

ReadResponse rr = proxy.get(request);

SalesOrder so = rr.record as SalesOrder;

SalesOrder updSo = new SalesOrder();