All I think is, what would the Sheriff have to offer Zelena? Perhaps inside information about Robin Hood? Perhaps this is how Zelena was able to better blend in as Marian by getting info from the Sheriff (all speculation at this point!)
That is a good theory but what if the sheriff had a magical itam that could revial robin's plan, and zelena needs that to stop him from steeling the magical potion so they would then both want robin gone and then start teaming up. and also rumple could have disgused him so the witch didn't see him coming (tom ellis in lacy) but didn't want to go through with it. also by traviling to oz, all magic comes with a price and robin said it was thanks to him that marian was lost so that deal with rumple could have caused marrian to be lost.
Lol, The Sheriff will likely only have dealings with Zelena if she comes to Sherwood Forest to get revenge on Robin; however, just because he's an antagonist doesn't mean he's working for or with Zelena, just as he didn't really work for Rumple when he went in search of Robin Hood.