Does the debt fall away from the son —who used to give his mother a monthly sum— if he discovers that the money he borrowed from his grandmother to cover his university tuition fees was originally for his mother, who refuses to waive the debt, even though she paid for his brother’s entire education?

The father is usually loving and compassionate towards his child, and what children imagine to be otherwise are delusions. Even if the mother falls short or prefers some children, her great right to dutifulness and kindness does not diminish. Therefore, you must be dutiful and kind to your mother in all circumstances. University tuition fees are not an obligatory expense for the father; rather, it is a donation and a benevolent act. It is obligatory to treat children equally in giving gifts, unless there is a need. If she exclusively funds your brother, that does not invalidate your debt to your grandmother, and you must repay the debt when you are able. You may politely and wisely ask her to be fair.