What is the legal (Shar'i) position of the seller, and is the sale valid in the case of selling non-transferable company shares, then the buyer demands a refund after the project is withdrawn from the company?

There is no objection to purchasing company shares before the company's establishment. There appears to be no impediment to the validity of their sale, and the contract is not required to be rescinded by Islamic law. The fact that the company's regulations do not permit the transfer of ownership of these shares does not affect the validity of their sale under Islamic law.