Let Free/Release Eromon is an option under the Eromon tab where the player may let Eromon free in exchange for in-game cash. Any Eromon may be let free, except for Eromon that has been locked for safe keeping and stored in the storage. Once an Eromon is let free, the released eromon will be gone. If the released eromon is available in gacha, fortune, etc., the player may get it back but with the starting level again.
This is one easy way to get cash when you lack some cash for Upgrade Move. The cash you may get differs from each eromons, presumably from the stats. Higher stats will fetch higher price while most N rarity eromons have the same price.