You haven’t given enough detail to understand your problem completely. But your reference to a prepaid sales liability account suggests you are reading forum postings. These could be obsolete, as the way Manager handles excess payments, customer credits, and deposits has changed over time. You should instead be reading this Guide: https://www.manager.io/guides/how-to/customer-deposits-and-advances.
As for your control account experiment, Customers are assigned by default to the Accounts receivalbe control account. They can only be assigned to one control account at a time. So for your Prepaid sales control account to work as you expect, you would need to edit each Customer individually, changing the assigned control account. But if you did that, your Accounts receivable functionality would be broken.
This inability to have Customers belong to more than one control account is why the entire process for entering advance deposits was changed last July: [16.7.73] Categorization against customer and supplier accounts more intuitive.
So…re-enter the deposit as described in the Guide linked above. You can get rid of your Prepaid income account (after reallocating any transactions that reference it). Your customer’s account balance will be in surplus. You can handle that according to this Guide: https://www.manager.io/guides/how-to/processing-refund-for-customer. If the customer is likely to buy again soon and agrees, you can just leave the overpayment in their account.