QB Issue Resolution:

enter the payment and use accounts payable as the expense (reason) for the payment, that will set a supplier credit.  Enter the bill when it comes in and then in pay bills, apply the credit and pay the balance if any.

Resolution for Issue 'How do I record a payment to a supplier when I am still awaiting their invoice ?' available: Yes (Solved).
Source: Intuit Community forum.

How do I record a payment to a supplier when I am still awaiting their invoice ?: this 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 issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

Support for Accounts Payable
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Error Recovery for Accounts Payable [Canada]
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