If you invoice by modules on the invoice, there is no 7% or 21% withholding, only the amount and VAT.
Billing by modules is a bargain as long as you do not get too much attention. The work of an SAP consultant does not fit into any module, so it is committing an illegality.

Not all agencies and companies accept invoices without withholding.
It has always been said that "Hacienda IVA" does not cross the data with "Hacienda IRPF" but the day it does it is going to take more than one ahead.
No member of ACISAP bills for modules because it does not consider it ethical.