Editing custom fields of an SO with API hits customer's credit limit?

Editing custom fields of an SO with API hits customer's credit limit?

Hi, I'm using API to edit a custom field of an SO (with tracking URL so customers can click it in their portal), but I get error "Sales order amount cannot be more than the customer's credit limit".

It looks like it's the same in UI. Instead of just editing the field, it looks like it's re-creating the whole SO, and (of course) it can hit credit limit.

Is there a way to update the SO without this? All I need I just to change one (custom) field.