If I set the value of "Item price" in one currency, and the value of "Sell-back price" in another currency, which is less valuable in relation to the purchase currency, then why can't I set a higher value? This is a very strange logic for a multi-currency plugin.
In addition, if the value of "Sell-back price" is higher than the value of "Item price", saving occurs without an error message, and the values become equal. This is not normal, there should be an error notification if You claim that was intended.