Adding collection charges to a debt

السلام عليكم ورحمة الله وبركاته
Mufti saheb I hope you are well.
May i ask a question?
I have some patients who do not pay their bills after receiving service.
Eventually, they get handed over to a debt collector.
The debt collector charges me 23% over the amount recovered ie if the amount is R100, I have to pay R23 to them.
Is it halaal to pass this cost to the patient? In other words, the debt collector collects R123, I pay him his R23 and gives me my original R100? Thus I only get the amount originally charged, of R100.
Or is this regarded as interest?
The collector also charges the patient an additional amount of his own.
I disclose to the patient that any charges are for his account from the start, and they are reminded of this in a final demand before being handed over.
جزاكم الله خيرا

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