You want the inverse perspective transform. If your original transform is S->S', you want the transform matrix S'->S
Mat InversewarpPerspectiveMatrix = getPerspectiveTransform(dst_vertices, src_vertices);
Then you make a SPARSE matrix
Mat PerspectiveCoordinates containing the vector x,y.
Finally you want to call
PerspectiveTransform(PerspectiveCoordinates,OriginalCoordinates,InversewarpPerspectiveMatrix)