Is it permissible for a customer to buy vegetables for the value of 300 Riyals, on the condition that he carries as much as he can in one go, given the variation in people's ability to carry?

Selling "gharar" (ambiguity/excessive risk) is forbidden and invalid. It refers to a sale where the item being sold is unknown, as the Messenger of Allah, peace and blessings be upon him, prohibited it. As stated in Sahih Muslim on the authority of Abu Hurairah: "The Messenger of Allah, peace and blessings be upon him, forbade the sale of gharar." Among the conditions for a valid sale is that both the item sold and its price must be known with a knowledge that prevents dispute, as ignorance in this matter leads to contention and invalidates the sale. Gharar is ambiguity, risk, and anything that leads to dispute, such as selling something that does not exist or is unknown.