What is the Sharia ruling on a brother reneging on his promise to sell his share of his father's house in exchange for two kirats of agricultural land, despite knowing that its value is less, considering it a gift from him, and then refusing the sale after the father's death when another brother offered a higher price? And what is the ruling on the questioner spending money on his brother's share, relying on this promise?

Fulfilling a promise is a religious requirement. The preferred opinion is that if someone makes a promise to another, and the one promised, because of that promise, enters into a difficult situation or undertakes an action, then the promisor is obligated to fulfill it, unless there is a valid excuse. In the event of non-fulfillment without an excuse, the promisor is required either to execute the promise or to compensate for the actual damage incurred because of it.