At first glance, this seems like a simple syntax problem. To get the currency
of the neighbor, you should be using of
:
set currency round ((currency + [ currency ] of neighbor) / 2)
But since you also said that you want both turtles to get the new amount, you also need to add:
ask neighbor [ set currency [ currency ] of myself ]
Or, perhaps less confusingly you could do something like:
set new-amount round ((currency + [ currency ] of neighbor) / 2)
set currency new-amount
ask neighbor [ set currency new-amount ]
One last variant, slightly better in my opinion because more general (and perhaps even clearer) is:
let sharers (turtle-set self one-of bystanders-here)
let new-amount mean [ currency ] of sharers
ask sharers [ set currency new-amount ]
In this last one, you don't even need the if neighbor != nobody
check because if there are no bystanders, turtle-set
will build an agentset containing only self
and the mean currency of that set will just be the present currency value.