There are two issues here, duty bound and legally bound.
I am not a lawyer, but if your uncle signed a legally binding agreement to sell the land subject to planning being received, then you are legally obliged to sell the land.
If it is the same as a house purchase, unless you have a signed contract, you are not legally obliged to sell it.
If your uncle genuinely agreed to sell it at a fixed price, but had no legal agreement in place, then it is really your decision. The key question here is what would your uncle have done? If he would have sold it, then you should probably do so. Especially if the costs of planning were paid by the potential purchaser.
Brendan