Three men snatched cash from a till after asking the shop assistant to change two £10 notes.
They then ran out of the Argos superstore, at the Fairacres industrial estate, in Abingdon, to a silver car where another man was waiting.
The offenders, who were all Asian, bought an item and then asked the cashier to open the till to change the money at 3.45pm on Tuesday.
Pc Philip Hampton said: "We are particularly keen to speak to a lady who took down the registration of the car, as she left before police arrived."
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