If they don’t want to use it for the agreed upon purpose, they could either offer to pay the true value so they can use it for something else or give it back to the farmer/heirs.

The real problem seems to be one city gave the land to a parks nonprofit who then sold it to another city, but the original park intent did not follow those sales.

And if they need it for something else they could just compulsorily acquire it from themselves for 10 more dollars?