If you're looking to move tokens from the Ethereum network, or any other EVM-compatible network, back to BSC, then you're in luck:
Binance has written a support article specifically to help you out with this.
Keep in mind that MetaMask simply allows you to access the Binance network, and has nothing to do with how tokens work, or get transferred onto or off of BSC. For this reason, we encourage you to consult directly with Binance's support resources for all Binance-related issues.
Make sure to ask Binance for any special requirements before making a transaction (for example, it has been reported that users need to set the Gas Price to 10 Gwei when making a transaction on the Binance Smart Chain).
If you have any further questions regarding Binance Smart Chain in general, or Binance Chain, please contact the Binance Support team directly.