If the Giants had given San Jose back to the A’s, that stadium would have been built and there would be no talk of the A’s moving to Las Vegas.
I don't understand why it needs to be "given" back. Haas offered the Giants the San Jose area to the Giants. (Even though it presumably would help him if the A's were the only team in the Bay Area) But the Giants obviously didn't move to San Jose, in fact, they moved farther away.)
So, in what way would the Giants still own the San Jose rights? It was unjust enrichment? They took something from Haas without consideration.