Ouail,
1) Read XML file using open dataset (because it's in background)
2) Download files to program memory
3) Get the invoice number (example, next 10 chars after <InvoiceNumber>... or between <InvoiceNumber> and </InvoiceNumber> from these 2 files
4) Check if invoice number isn't char (in case that when initial the next characters are the closing tag).
5) Compare both invoice values from xml files
6) Build the text email with the compare detail. "...test is not ok... / .... Teste is ok...."