The act of keeping the extra $20 implies that the person is dishonest as they found money that doesn't belong to them and instead of returning it, they decided to keep it for themselves and hence should not feel pride for such an act
Consider the statement "I went the store to get food and got back $20 extra in change from the cashier and ended up keeping the money.". Based on this "Went to the store to buy some food and came back with an extra $20 that I originally should have returned and now I feel so proud of myself for having kept the money" a true statement? Yes, no, or I don't know?