This happened to me with a purchase of £1500 of travel money!!
I bought £800 of Mexican money from Debenhams and got a transaction code. I then tried to put through another order of £1500 of dollars. This time got a transaction failed message. But when I checked my bank details, the money had left my account.
Rang the bank in the morning, they said I must have used a fraudulent website and I'd been conned out of the money. That was very helpful! I explained that I knew I hadn't, so their advice was to phone Debenhams
I phoned Debenhams who explained that it could see the transaction - it had been declined because you're not allowed to buy more than £1500 in foreign currency per 24 hours. They said they hadn't taken the money, contact my bank.
My banks said they had sent the money; contact Debenhams.
Debenhams said yes, the bank had sent the money, but they hadn't clamed it; contact your bank
The bank said they had sent the money, ask Debenhams to cancel the transactions.
Debenhams said, we haven't claimed the money; contact your bank!!!
I went round and round in circles for 10 phone calls until I was told by both ot them, there was nothing they could do. I had to wait up to 5 working days for the money to drop back into my account!!
Apparently, the bank couldn't refund me because they only had my word that I'd not had the goods the money had been debited for. Debenhams couldn't refund me as they had declined the transaction and so had already sent the money back to the bank I just had to wait for the refund to go back to the bank and then back into my account.
So, with 7 days to go until my holiday, my holiday spending money was caught up in cyberspace for 6 days!
And yes, I know it's not ideal to take so much currency but there were reasons for it