Three men snatched cash from a till after asking the shop assistant to change two £10 notes.
They ran out of the Argos superstore, Fairacres industrial estate, Abingdon, to a silver car where another man was waiting.
The offenders, who were all Asian, bought an item before asking the cashier to open the till to change the money at 3.45pm on Tuesday.
PC Philip Hampton said: "We are particulalry keen to speak to a lady who took down the registration of the car to get in touch as she left before police arrived."
Anybody with information should call Abingdon police on 01235 556857 or the anonymous Crimestoppers number 0800 555111.
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