During the trust audit, I found that some accounts have been overcharged. I can not find a source if there is any tolerance or if difference must be refunded. Any help is appreciated.

Sounds like a breach of contract to me. I'll let you be the judge whether that should be cured or not.

We take one of two approaches depending on the account/client. Either way, we make the client whole. The first approach is to reimburse the account in full. The second is to waive all or partial amount of fees until the account is made whole.