Is a person sinful for refusing to return land that a relative sold to him, after its value increased, despite the relative's claim of a prior agreement to reclaim it when money became available, while the buyer's intention was to complete a fully constituted sale and purchase transaction?

What transpired between you and your aunt's husband is considered a "Bay' al-Wafaa" (sale of fidelity/trust), which is when a person in need sells a property on the condition that they can reclaim it upon returning the purchase price to the buyer. This type of sale is deemed invalid by the majority of scholars, including the Maliki and Hanbali schools, and the early Hanafi and Shafi'i jurists, for two reasons:

1. Its intent is to lead to, or results in, forbidden usury (riba), as it is a loan that yields a usurious increase in the form of benefiting from the sold item. 2. The seller's condition of reclaiming the sold item upon returning the price contradicts the very essence of a sale, which is the permanent transfer of ownership of the sold item to the buyer.

The Islamic Fiqh Academy has determined that the reality of this sale is a "loan that brings a benefit," and that it constitutes a circumvention of usury, and is therefore impermissible in Sharia.

Accordingly, the land remains the property of your aunt's husband, and he must return the price you paid him. The increase in the land's price holds no value.