How can I purify myself from the sin of losing a cell phone entrusted to me for sale and to donate its price as charity, given that the buyer did not pay the agreed-upon amount?

You are in the position of an agent for the owner of the mobile phone. You are not liable unless there is transgression or negligence. Since you handed over the sold item—the mobile phone—to the buyer without receiving the price, you have been negligent, and thus you become liable for its value. Therefore, you must give the value of the mobile phone in charity on behalf of its owner.