If a suitor is rejected for financial reasons despite there being acceptance and performing Istikhara, are there specific actions or supplications to make him one's husband in the Hereafter?

There is no specific supplication or particular action for this matter. However, it is hoped that the supplication of Dhul-Nun (Jonah) or that made with the Greatest Name of Allah will be answered. It is advised to turn your mind away from the rejected man after performing Istikhara if the family insists on not marrying him to you, and to humbly beseech Allah to grant you a righteous husband and good offspring in this world, and to unite you both in Paradise.