Is it permissible for him to sell the land which he swore not to sell unless he built a mosque upon it, in order to use its money to renovate another mosque, and does he owe expiation for a broken oath if he does so?

A man's oath not to sell his share of land unless he builds a mosque on it means that he swore not to sell it except if he builds a mosque on it. He can absolve himself from this oath by paying expiation, then sell his land and benefit from its price as he wishes. The expiation for an oath is mentioned in Allah's Almighty قول: "Allah will not impose blame upon you for what is unintentional in your oaths, but He will impose blame upon you for what you intended of oaths. So its expiation is the feeding of ten needy people from the average of that which you feed your [own] families or clothing them or the freeing of a slave. But whoever does not find [anything] - then [it is] fasting for three days. That is the expiation for your oaths when you have sworn" [Al-Ma'idah:89].