If the client's debt doesn't have an associated receipt or invoice yet, you can cancel the service that is causing the debt.
To do so go to the client's file and look for the service.
*For example, if it's a membership fee, will be in the subscriptions area of the client's file.
If you're not sure what service caused the debt just go to billing and collection, and once there click on the debt tab.
You'll see the description of each debt so you can know where it came from.
When you found the service causing the debt remember to select "All" in the status drop down menu so you can see all the services and not only the current ones, cause the debt might come from a non-current service and if you don't select "All" you won't see it.
Once you located the debt press 
on the service to see all the payments and debts of that service. then you can see if the debt you are looking to remove already has a receipt or invoice created or if it doesn't.
We have a few options here to erase the debt or manage the payments and debts:
This button creates a receipt or invoice if it's not created already, but won't collect the amount, only do the document.
This button associates a payment (that has a receipt or invoice) with the fee. For example, if we pay the service in a different way, through the cashbox creating a manual receipt, we can after that associate it with the fee pressing this button.
This button will undo the previous one and also if we have a receipt and a payment associated with this already will disassociate it (but the receipt and the payment will still exist).
This button is the first option to delete the debt, only works if there is no receipt or invoice for the fee already. If you press it you'll set the debt to 0 without creating a receipt or invoice, and no cash movement at all.
This is to pay the fee, it will take you to the payment options.
If you press this button you can see more options, like change the amount of the fee, the document you want (receipt or invoice), change the payment method...
This button prints the receipt or invoice of the fee.
And this is the last option to delete the debt. If you press this button you erase the fee, so the debt disappears.
In some services, you won't have all these options, but you should always have the 
to delete the fee.
If the debt already has an invoice or receipt, then you have to delete it in other way, cause you shouldn't erase a legal document that is an invoice or a receipt.
If this is the case, here you have a link to show you how to do it.