How to address the unfulfilled demand when buyer reduces the quantity in PO

We have a BPA with line item A, agreement quantity is given as 10. BPA is enabled for touchless buying.

Requester creates a catalog requisition for item A, 15 quantity and submit for approval. Once it gets approved, application will automatically create the PO in incomplete status but with a validation error. The error states that "Your order will cause the total quantity released against agreement '****', line 1 to exceed its quantity limit". As a next course of action buyer will edit the PO and reduce the quantity to 10, then submit for approval.

Here there is an unfulfilled demand of 5 quantity raised by requester, that is not available in the process requisition. How we will address the unfulfilled demand. How should we handle this case. Kindly guide me.