Changing the line vat amount on an invoice using the di api SBO 2005a

Hi

I am using SBO 2005a SP01 PL36.

I have written some code to import sales invoices which works fine apart from the fact I want to overwrite the line vat amount -

oinv.Lines.NetTaxAmount = vatam

SAP seems to ignore this and just calculate it anyway, it's like it's a read only field

Can anyone help me please ?

Thanks

Regards Andy