Hi Andreas.
Instead of 'inventory_quantity' , use quantity just like:

 self.env['stock.quant'].sudo().create({
    'product_id':product_rec.id,
    'location_id': location_rec.id,
    'quantity':product_qty
    })


But this is not the valid way for stock increase technique. Use a Odoo standard way for updating the stock. Please check the update quantity functionality from odoo base code.

For your internal programming, Hope it will help you.