Tax Exemption per job instead of Customer

We have this builder who does many jobs most taxable some non taxable. Back when we started them in IO were told to the tax exemption goes with the customer so we put Two Pfund Constructions in IO. One taxable the other Non Taxable with the apropriate exemption letter for that particular job. As different non taxable jobs come in the current non tax letter does not apply. Is there a way to make tax exemption follow the individual job or deliver to Projecdt and attach the exemption to the particular job instead of the customer?