If the Russians pay $1,000 worth of rubles today, but that payment is stored in a bank until the sanctions are lifted, at which point same rubles are worth $500 dollars, then the Russians gain the difference. Because when they made the deposit it was at the right value. Conversely, if they wait until the sanctions are lifted to pay back the debt, then they would have to pay back many more total rubles because of the rubles decline in value.
If the Russians pay $1,000 worth of rubles today, but that payment is stored in a bank until the sanctions are lifted, at which point same rubles are worth $500 dollars, then the Russians gain the difference. Because when they made the deposit it was at the right value. Conversely, if they wait until the sanctions are lifted to pay back the debt, then they would have to pay back many more total rubles because of the rubles decline in value.