MIRO records a supplier invoice with reference to a purchase order. The reference connects the supplier's demand for payment with the price agreed on the PO and the quantities received by the warehouse. For a normal matched invoice, the posting debits GR/IR and credits the supplier payable. Stock doesn't rise again when you enter the invoice.
Read the supplier's invoice reference carefully. Reusing the same reference for that supplier can create a duplicate payment risk, so the training client rejects it. Quantity and price differences are separate checks. An invoice can post and still be blocked for payment, which means a saved document number alone doesn't prove it is ready to pay.