Cant reverse pending funds

A while back i cancelled online purchase. Phoned fnb and they were able to release the funds back into my account as it was still pending.

Lastnight i made payment on airbnb,cancelled this morning and airbnb advised that the refund is immediate on their side,but fnb tells me they cant do immediate release yet the funds are still pending.

Well thats very dissappointing as i cant afford to wait 10days for that transaction to be refunded.