18. Where a person contracts to sell or let certain property, having only an imperfect title thereto, the purchaser or lessee (except as otherwise provide by this Chapter) has the following rights:-(a) if the vendor or lessor has subsequently to the sale or lease acquired any interest in the property, the purchaser or lessee may compel him to make good the contract out of such interest;(b) where the concurrence of other persons is necessary to validate the title, and they are bound to convey at the vendor's or lessor's request, the purchaser or lessee may compel him to procure such concurrence;(c) where the vendor professes to sell unincumbered property, but the property is mortgaged for an amount not exceeding the purchase-money, and the vendor has in fact only a right to redeem it, the purchaser may compel him to redeem the mortgage and to obtain a conveyance from the mortgagee ;(d) where the vendor or lessor sues for specific performance of the contract, and the suit is dismissed on the ground of his imperfect title, the defendant has a right to a return of his deposit (if any) with interest thereon, to his costs of the suit, and to a lien for such deposit, interest and costs on the interest of the vendor or lessor in the property agreed to be sold or let.