Is it forbidden to sell an old ring and buy another of lesser weight, paying the difference, without first receiving the price of the old ring? And what is the way out of this situation?

As long as you sold your old gold to a merchant and bought other gold from him, and you did not receive the price of the first in cash, but rather an offset was made between the two prices and the difference was paid, we hope that there is no problem in that, because the possession occurred through offset, which is legally sufficient, even though actual possession is preferable. You are not obligated to sell or return the gold, but it is preferable in the future to receive the price of the old gold first before buying other gold.

Even if the transaction was an exchange, not a sale, and all the gold is fashioned and returning it is difficult, one can adopt the opinion of those who say that fashioned jewelry, through its fashioning, transforms into a commodity. Thus, it is permissible to exchange it with an increase or delay, and the rules of currency do not apply to it. This opinion is based on the idea that fashioning makes jewelry a commodity, not currency, so usury does not apply to it, just as it does not apply to other commodities. Although this opinion is less preferred, it can be adopted when necessary to avoid hardship and when returning is impossible, as the general rule is to act upon the preferred opinion unless there is a necessity or need that permits adopting the less preferred one.