HandleStockChange.cs

public async Task Handle(ProductStockChanged stockChangedEvent)
{
    var product = await productRepository.GetBy(stockChangedEvent.ProductId);
    var purchaseOrderLine = await purchaseOrderLineRepository.GetBy(stockChangedEvent.PurchaseOrderLineId);
    
    // validation...
    
    if (stockChangedEvent.IsStockRaised)
    {    
        // OOPSIES ⬇️
        product.Stock += stockChangedEvent.Quantity;
        purchaseOrderLine.QuantityDelivered += stockChangedEvent.Quantity;
        
        await purchaseOrderLineRepository.Update(purchaseOrderLine);
        await productRepository.Save(product);
    }
    else
        product.Stock -= stockChangedEvent.Quantity; // stock decrements don't affect purchase deliveries
    
    // trigger a use case that depends on updated product and purchase order line    
}