Question on Contracts LAW

John had just started a business selling smartphone accessories.  He had his stall set up in the city where his stall had displayed all his goods for sale.  One Friday, Jill, upon visiting John’s stall found a portable charger she needs for her smartphone and asked for the price to which John responded that it cost $20. She then replied,” OK, but currently, I do not have the cash with me but I will come by on Tuesday to pick it up.” John responded was “Don’t worry; I will be here every day next week.”

On Tuesday, Jill goes up to John and says, “This is $20 dollars for the portable charger.” However to her disbelief, John told her that the price has increased to $30 over the weekend and that he would not sell it to her at the previous price of $20.”

Advise Jill whether she is able to legally force John to sell the charger at $20.