If B is making a payment, just receive the payment against the invoice for A

Other wise why would you want to transfer the sales invoice?  Does B have a customer credit on the books that is going to be used to pay it?

Resolution for 'balance transfer issue: 'customer B wants to pay Customer A Balance, how A balance transfer to B' available: Yes.

The balance transfer issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the balance transfer issue yourself by looking for a resolution described in the article. If it is a complex balance transfer issue or you are unable to solve the issue, you may contact us by clicking here or by using other balance transfer support options.

For more information about resolving issues related to balance transfer in Quickbooks/QBO or to get support for the issue 'customer B wants to pay Customer A Balance, how A balance transfer to B', click the link below: