A client refuses to pay a small amount. What to do?

Hi,

We are a medical clinic where a patient is not paying a very small amount of money, 5000 AED, even she has signed informed consents and promissory note that she will pay but she didn’t.

The problem is that it is not the first time to happen to us. Should we file a case with the police?

Should we send a legal notice? Or what can we do?

We called her several times and either she refuses the calls or says that she will pay.