"Facturées à l'euro l'euro" would be something like "invoiced at cost". Some disbursements/expenses can be recharged/passed back to the client without any additional margin/taxes/added value. Grossly generalising, any number invoiced to a client should have VAT/TVA slapped on top, and this is an exception. The magic phrase in French is "à l'euro l'euro" and I think the BE equivalent is "at cost". So, the original sentence would be something like "withheld taxes will be invoiced at cost".