What is the ruling on someone who guarantees one party in a sale operation, and then both parties ask him to be relieved of responsibility due to their mutual trust? And does one legally have to pay a debt to a company if the value of the shares owned by the debtor in that company has lost their value and become close to zero?

The absolving of a guarantor by the right-holder is a dissolution of the guarantee and a release from the suretyship. So, if the seller absolves the guarantor from his guarantee for the buyer regarding the price, the guarantor is released. As for the debt owed by a shareholder to the company, it is not waived in exchange for the shares he owns. Shareholders bear losses commensurate with their shares. If the loss is due to the director's negligence, then he is liable to guarantee the shareholders' funds from his personal wealth.