Is the action of the Chairman of the Board of Directors—modifying the commercial register so that the right to sign becomes his alone, reserving lands in the company's name without the partners' knowledge, and then refusing to give them their share in it after they became aware and were ready to pay—considered haram and an usurpation of the partners' rights?

Modifying the commercial register without the permission of the two partners is considered a betrayal and requires repentance and restoring the matter to its original state or reaching a mutual agreement with them. As for the lands allocated to the company, they belong to the company if the two partners agree to that and pay you their share of the price. Otherwise, you must inform the designated authority about the matter, because the allocation was for the company under specific conditions and not for you personally. The conditions must be adhered to, or the granting authority must be consulted.