How is a gold belt loan, which was sold by his wife for less than its perceived value, repaid without falling into the suspicion of usury?

The gold loan must be repaid in the same weight, not by its value at the time of the loan. When repaying, your wife can receive her due in paper currency, provided there is a spot exchange and according to the price of gold on the day of repayment, unless she accepts less or you donate more.