Is the salary earned from a ready-mix concrete company that cheats on the quantities of concrete and cement, issues fake invoices, and falsifies samples, considered permissible (halal) or forbidden (haram)?

If your work at the company is unlawful, then the earnings from it are unlawful. If it is permissible, then the salary is lawful. However, you must advise the officials to cease the unlawful activities; otherwise, you must inform the customers about it. If your work is permissible but you assist in something unlawful, then your salary contains a prohibited element to that extent.