You bought a bike. They ran a sale. You went back and asked if they would take the sale price for a bike you had already bought. They gave it to you. Not one single thing wrong with this. In no way can it be conscrued as you ripping them off when they voluntarily gave you the difference in price.

What it is, is good customer relations. A good businessman sees this as insurance. I'd say there is a better than excellent chance you'll be back there and spend more than $50 in the near future. In fact, you already have.