Custom Price Books can impact the Partner Billing Rule description.
For example, when a Billing Rule contains a description with a formatted currency string "1,750", and a Custom Price Book is configured to replace that price with "-1750", the description replacement logic will impact the final number string.
Instead of replacing the entire amount, the system identifies the comma (,) as a delimiter or terminating character. This results in the new price being prepended to the remainder of the old price, creating the string "1750,750".
Removing the comma (,) from the Billing Rule description will allow the Custom Price Book logic to correctly replace the string with the expected value.