Libby Hamilton, 33, who owns BWNAB Bakery, also known as 'the pink shop' on Handsworth Road, said she went to Tesco Extra on Saville Street on Friday evening when the incident happened.
In a Facebook post, she said she lost her handbag 'that had all the money I had left for Christmas' at around 7pm, after she walked out of the supermarket and packed her shopping into her boot.
She then accidentally left her bag on the trolley, only to realise it was not with her 20 minutes later.
She said: "When I went back, the shop was shut. After spending 25 minutes attracting staff attention, security checked the cameras.
"They can see me leave the shop, unpack my shopping and take my bag back to the trolley bay, leaving the bag, and then, 20 seconds after I leave, a car stops at the bay, a person gets out, takes my bag, gets back into the car and leaves.
"Conveniently, it is not clear enough for a register plate. "
‘I hope that whoever took it has a lovely Christmas with all my money’
The mum-of-four said she was left 'gutted' as it was only three months since her bakery was launched and that she was still paying back the cost.
She explained, "Money made at the shop goes back to the shop for wages and running costs as it is set up as a company.
"As most people say, the first years are the hardest, and I rarely take a wage after outgoings, so I had to work really hard to save that money up.
"I am absolutely gutted to say the least, and I hope that whoever took it has a lovely Christmas with all my money.
"I will never ever use cash ever again, and I will never bother with a handbag. I don't usually use one and really don't know what made me grab one to use today."
Bakery to open for one day
Libby's post then attracted hundreds of comments from her customers and members of the public, who then offered to help find her handbag.
Some even wanted to contribute some money to replace the money she had lost.
She said: "People offered money, but in a million years, I'd never take it! We've had a crazy busy three months since opening on September 18, so we were planning to have Christmas and New Year off. "
But after the incident, she has now decided to open the shop for a day in between and try to recoup some of the costs by doing what she knows best, which is baking.
She added: "My customers are amazing and really loyal as many of them have watched me grow from doing this part-time at home to full-time at home and then to a shop, which has taken 10 years and I'm still trying to grow."
Libby said the bakery is now offering a New Year treat box for £15 that contains two soft vanilla cupcakes, two rich Nutella-stuffed cookies, and two fudgy brownies. Collection is from noon to 3pm on January 31 from the shop, 122 Handsworth Road, S9 4AE.
To order, please visit www.bwnabbakery.com.