House of fraser refund refused.

just went into house of fraser bought 3 things,asked if store price was same as online. Was told yes and got overcharged £15. Queried it and was told i could recieve a credit not. Spoke to the manager who said i could have a credit note or nothing. walked out with a credit note,i told him vou cant give a credit not for a overcharge,he said they could. im 100% sure they cant do this.
had to walk out,was going to hit the roof.

Is this right ?