A person' s failure to keep a promise is wrong only if, first, doing so harms the one to whom the promise is made and, second, all of those who discover the failure to keep the promise lose confidence in the person' s ability to keep promises
Because he lost his job, Carlo was unable to repay the money he promised to Miriam. However, Miriam did not need this money nor did she lose confidence in Carlo's ability to keep promises. So, Carlo's failure to keep his promise to Miriam was not wrong.