. Pay me back the money you took or I will tell the police. If you…………………………….……………………………  

If you do not return the money you took, I will have to inform the police.

In case you don't pay me back the money you took, I will tell the police.

Should you not pay me back the money you took, I will tell the police.

If you fail to pay me back the money you took, I will tell the police.

Unless you pay me back the money you took, I will tell the police.