What is the expiation for someone who swore an oath by divorce that he would not build on a plot of land he owns, but then wanted to build on it for his wife? And is it permissible to circumvent the oath by selling the land to his wife so that she can undertake its construction?

Swearing by divorce is subject to the ruling of conditional divorce. If the swearer does what he swore to abandon, divorce occurs according to the majority of scholars, whether he intended to bring it about or not. Some scholars hold that an expiation for an oath is due if the husband did not intend divorce. If there was a reason for abstaining from building, and that reason ceased to exist, then building does not constitute a breach of the oath, unless the intention was to never build at all. Selling land to the wife for building on it refers back to the swearer's intention at the time of swearing. Thus, the reason and intention have an impact on the oath.