Is a phone agreement to sell a car and determine its price before inspection and examination considered a binding contract, or is the car owner allowed to withdraw from it if he receives a higher offer?

Concluding contracts over the phone is valid and permissible. Contracts are valid via phone or similar means for transactions that do not require immediate physical possession. However, for transactions that do require immediate physical possession, they are valid over the phone if possession takes place immediately after the conversation ends. Selling a commodity over the phone falls under the category of selling an absent item, which is valid with the option of annulment (khiyar) remaining for the buyer, provided it is not merely a non-binding promise.