Is buying goods for one's sister with money taken from their mother, and then the sister returning the original amount to the mother, considered usury (riba)? And is exchanging one usurious commodity for two usurious commodities forbidden, like exchanging gold for gold and silver?

The transaction mentioned between the mother and her sister is not usury (riba) because it is a loan of one thousand, where the questioner was appointed to purchase an item for the sister using the loan amount, then returned the remaining portion of the loan to her. Afterwards, the sister appointed him to deliver an amount equivalent to the loan to the mother. Thus, she has repaid one thousand with one thousand.