This is a very common scam which is often run out of Nigeria and it doesn't just apply to rental propertues but anything you could be selling, cars, consumer goods etc
The way it works is that they do send a payment, normally by cheque on a fireigmn bank which then appears in you bank account, for a larger amount and it appears in the bank account but it is uncleared
So you ask for €100, they send you €500, say that they made a mistake and ask you to return the difference
The buyers will then ask you to send the amount of the overpayment back to them, normally but a Western Union transfer and rely on the time difference between the cheque being rejected and received, the payment eventually bounces and you are left out of pocket
In the Summer we proably get one of these every few weeks