Richard sold a large parcel of land in Cebu to Leo for P100million payable in annual installments over a period of tenyears, but title will remain with Richard until the purchaseprice is fully paid. To enable Leo to pay the price, Richardgave him a power-of-attorney authorizing him to subdividethe land, sell the individual lots, and deliver the proceeds toRichard, to be applied to the purchase price. Five years later,Richard revoked the power of attorney and took over thesale of the subdivision lots himself. Is the revocation valid ornot? Why?