I think their debts for each other are much deeper than simply saving each other's lives
Her debt to him isn't only about how he made a different call when he was sent to kill her. They are partners for years. They have been to numerous missions together and have each other's back every time. They have shared their deepest thoughts and darkest past with each other. They show each other that they are more than just some killing machines. They prove to each other that they are not just Hawkeye and Black Widow, they are Clint and Natasha to each other. They basically owe each other everything