John took out a loan with his bank to buy his car. He agreed to pay back $$10,000$ plus $3\%$ interest accrued annually over the next $4$ years. Three years later he runs into financial difficulties. He still owes $$2,800$ on the loan. He asks the bank if they would accept $$2,100$ for the loan and they agree. Once John pays the $$2,100,$ under the rule used in the majority of state, could the bank come back later and demand the full amount of the original loan? Explain your answer.