For noobs testing LN with amounts that small, I think it’s a much better UX to just use a custodial LN wallet like CoinOS or Wallet of Satoshi. Those 500 sats are custodial in Muun anyway since it’s too small to settle on chain.

You can also send the sats on-chain with CoinOS with no fee other than the network fee. It’s cheaper than Muun even especially for larger amounts.