Two fake notes were tendered at the Shelter Scotland store in the High Street at about 2 pm on Monday.
The first was accepted but the second was checked and found to be a hoax.
Now police are searching for the two women behind the scam.
The first is described as about 34 years old, of average build, with brown hair in a pony tail and an English accent, while the other was aged about 20, of average build, also with brown hair in a pony tail and spoke with an English accent too.
PC Lee-Ann Hawker at Dumfries said: “It appears that these two women were quite happy to try and defraud a charity shop in the town after tendering counterfeit notes. We ask everybody to be on their guard for counterfeit notes. Everyone expects high value notes to be checked no and no-one seems embarrassed by this process.”
Anyone who can help identify these two women is asked to call Police Scotland at Dumfries in the 101 number.