Donate for my overpayment

hi, i'm a student from singapore. recently, i worked at this fast food restaurant. i recently got my salary and i didn't know that it was overpaid. so i've used most of it for my school. firstly, they didn't tell me that it was overpaid, amd secondly, they told me a month after i got my salary. it's 489.80 dollars and idk where i have to get the money