I don't think you can just ring up and tell people you can shift their freight today as our loads are low, but sorry we couldn't do it last week.
You generally would be locked into contracts to provide X amount of freight capacity a service / week. When full of pax, weights mean there is not too much capacity for freight on the 380 Mel/Dxb direct.
There would be a small market for short term freight, but not enough to justify the 200+ tonnes of fuel for a half empty load.