When moving ERC20 assets from Ethereum to Avalanche, you are allowed to transfer any number of tokens you would like to. The bridge was designed in such a way that minimizes transaction fees. However, if the transaction fee is higher than the value you are looking to transfer, it may make sense to wait until the Ethereum gas price decreases.
When moving assets from Avalanche back to Ethereum, the bridge charges an in-kind transfer fee, as described here. The user interface does now allow transfers less than the fee amount. If a user manually generates and issues such a transaction, the bridge will mark the transfer as invalid and not process it.