Is it permissible to take a small amount of money from one's brother without his knowledge, then inform him and consider it a debt to be repaid later, by estimating the amount and pardoning the mistake if the original amount is forgotten, or is this considered usury (riba)?

If your brother gives you permission to take from his money without his knowledge, there is no harm, provided you know the amount of the debt. If the amount is forgotten and you both agree upon a certain sum, then there is no harm. However, if he does not give permission, it is not permissible to take. If you have taken, you must return what was taken. If its value is unknown, you must strive and act based on what you believe to be most likely, or agree upon a certain sum.