Rebaz Haji, 33, noticed a £20 note was stuck on the glass door when the load had finished.
He started pulling out shirts, jackets and trousers and counted a total of £540 in £20 notes.
Rebaz and his wife Kwazhin Birot, 23, realised that none of the notes were damaged.
He said: “He left his clothes for a wash, dry and iron.
“He comes every week with two bags for a service wash.
“I put them in the machine and then all of a sudden the drum filled up with cash.
“None of it was damaged.
“I counted £500 all in £20 notes.
“Then I found another 40 quid I had missed at first.
“I did not know what to make of it.”
They rang the customer, who is a mechanic, from the laundrette in Hunslet, Leeds, West Yorks.
Rebaz said: “I rang him three or four times but he didn’t answer so I texted him.
“I said I’d found something in his clothes.
“He had no idea what I was talking about and he did not think he had left money in his pockets.”
The customer returned and rewarded Rebaz with £20 for his honesty.