What is the ruling on issuing sales invoices for building materials to a vocational institute, knowing that the institute does not receive the goods immediately but rather registers the amount as a balance to be used later for the institute's maintenance?

These employees are agents of the institute's administration, and therefore, it is not permissible for them to purchase anything other than what they have been instructed to buy. It is also not permissible for the shop owner to collude with them on this matter. The fictitious purchases they make hide an agenda that is not approved by the administration.