Well if Cora knows that her first born is already “lost” to her, then she can make the deal and get what she wants–the ability to spin–and later claim that the first born clause is negated and instead Rumple must seek out Zelena if he wants her first born. She’s being very shift and clever. She can promise him her first born, but he doesn’t realize she’s already had it. .
That is the part that is perplexing me, it is easy to see Cora attempting something like this to outsmart The Dark One, but why would she then go and change the deal. Was it because she didn’t want Rumple to know about her illegitimate offspring? Did she want to try and make amends for attempting to manipulate the man she now had feelings for? Was she afraid if Rumple found out about Zelena that she would be left alone with a child once again?
“The truth isn't easily pinned to a page. In the bathtub of history the truth is harder to hold than the soap and much more difficult to find.” - Terry Pratchett