Roland is right- the 28-50 will bring up the 50mm (and 75mm) frame lines in M2-M7 cameras. The frame lines are paired in one of three positions: 28/90, 35/135, and 50/75. If you have a 35mm lens and want to see the appropriate frame lines in a M2/M4/M5/M6 or M7 (or M8...) You will need a 35/135 adapter. M-mount lenses and LTM-M adapters have a tab of metal on the side which actuate the frame line mechanism in the left side of the mount. If you remove the lens from your M body, push the frame line selector lever, and look in the camera, you will see the thing moving. This needs to be in the right position to call up the correct frame line; this is part of the M mount, and requires the right adapter.
The old 28/50 adapters made for the M3 will work on any M camera as a 50/75 adapter, but this is still the wrong one for your 35mm lens. The salesman was right- it will work, and the lens should focus correctly (if it doesn't, it shouldn't have anything to do with the adapter), but it won't call up the right frame liens for the lens. I'd suggest you keep it and use it with other lenses in the future, and get a 35/135 for your 35mm lens. Good luck!