Am I entitled to demand the salary initially agreed upon, which was 9000 Riyals, and to be compensated for the previous period, considering the second agreement null and void?

If the agreement with the project manager was firm and unconditional, you are entitled to the agreed-upon salary (9000) until the end of the contract period. No other managers are permitted to change that, whether the agreement was verbal or written; for Allah Almighty says: ﴿O YOU WHO HAVE BELIEVED, FULFILL [ALL] CONTRACTS﴾, and His saying: ﴿AND FULFILL [EVERY] COVENANT. INDEED, EVERY COVENANT WILL BE QUESTIONED﴾, and the Prophet, peace and blessings be upon him, said: "Muslims are bound by their conditions."

If you accepted a lower salary under duress, you have the right to demand an adjustment to the contract and to receive the difference for the past period.

If the project manager was not authorized to conclude contracts, then the company is responsible for this error. The employee's right to his contract, which resulted in him leaving his previous job, is not forfeited, and the contract must be fulfilled for him until its end. Afterwards, the company can change the contract; if the employee agrees, then all is well, otherwise, the company is not obligated to anything.