Sram and Shimano cassette spacing is the same, so maybe you were tricked into buying a cassette you didn't need.

Possibly the mechanic was confused because while cassette spacing is the same, the cable response is different, so you cannot mix Shimano derailleurs and Sram levers.

There are also subtle differences in the cut of the teeth and shift gates between Shimano and Sram cassettes, but in any case Sram cassettes are commonly used with Shimano drive trains, both OEM and as replacements.