Sodastream

I have been using Sodastream for years. When my machine broke I bought a new one with a better mechanism (where the old one broke). The new one has a quick connect gas cylinder which is not compatible with the old. I phoned Sodastream who told me to take the old cylinders in for a refund of my deposit which was R150 plus. At the retailer the salesperson phoned Sodastream who only offer R10 on return. Surely a deposit is a deposit and a refund should be 100%? Not 7,5%?