I can’t change my amount here because the invoice has 1000sats attached to it. But there’s a guy who has zapped 21sats. How come? 
Discussion
You have to change the amount in the client.
The invoice is encoded with an amount of 1,000 sats. That amount cannot be changed, a new invoice has to be generated instead.
I’m using Damus, how can I change the invoice?
If you tap and hold the zap button ⚡️ Damus will show a pop up with different amounts. You can tap the amount at the top to change it.
It looks like this: