I can address this issue, but it may need to be something you explicitly configure, or else it will be a backwards incompatible change for existing users. The reason it works this way (skin is compared only when the currency is configured with a specific skin) is that the server owner could setup a vending machine that allows players to trade in their extra guns (which could have many different skins) for scrap, and the skin of the gun should ideally not be relevant in that case.
Given your special circumstances where the same base item is being used for the merchandise and the currency, I could special-case the logic to avoid taking currency of the same skin as the merchandise (i.e., accept any skin except the merchandise skin), or just force the unskinned version to be required. This approach would make it so you don't need to configure this (which would be great for anybody else trying to do something similar) and would minimize impact to existing users.
sorry for offtop, may be you can adaptive plugin for IQEconomic of multiple economics via HOOKS for balance in config? <3
I don't understand this request. Are you simply asking that the IQEconomic plugin be supported, similar to how the Economics plugin is supported?