You need to retrieve the Product via the association with OrderLine_Product in order to access any of it's attributes. This can be done using the "Retrieve" action and then "Retrieve via association", assuming the association between Product and OrderLine is already set.
Once you have the Product object in your microflow you can set the $OrderLine/Amount with a Change Object on the OrderLine and then set $Product/unitPrice*$OrderLine/quantity
Example: https://modelshare.mendix.com/models/ee345978-0691-4683-8b58-9289eb7d2bbe/order-line-set-price
That did the trick. Thank you very much.