The person stopping to listen has given attention before deciding about money.

Attention is the first thing offered and the one least often counted. Stopping for a busker means standing still in public, being seen to be interested, and letting the performance reach you, all of which happen before any coin is considered. That is a real gift, and it is frequently the one the performer values. Being listened to and being paid are separate transactions.

When you cannot give money, you can still stop, look, and say something afterwards. Notice too where you have been given attention lately and never registered it as generosity.