Bug: when orders get changed in different system, we can't fulfill in Saleor

[JannikZed]

For example: order gets created in Sleor with 1 pcs of item X, we change it in Saleor to 3 items - what to do? Should we update the order? Or should we just fulfill the orderlines from Saleor?
Furthermore, we should not fail on the error "{"field":"orderLineId","message":"Only 1 item remaining to fulfill.","code":"FULFILL_ORDER_LINE","orderLines":["T3JkZXJMaW5lOmRkZGExNjRjLWY4OGItNGU4ZC05Nzg1LWNiNTViYTJkYjQxYw=="]}",