College Workshop

Hey my college has conducted a workshop for us ,and we had to pay 500 to attend it ok,But it was not compulsory for everyone.the workshop is for 3 days( feb 6-feb 8).They sent us a Google form to register(They said now u all register,,later u guys decide whether to Pay/attend or not) and now that message is nowhere to be seen.Today morning we all got a message that whoever registered should pay 500,no matter if u r attending or not.Is this a scam????what should we do😭.