3d - How do I reverse-project 2D points on a plane into 3D?

Question

So, I have a plane P.

I've got the 3 coordinates of the plane P normal vector n(n1, n2, n3) and I've got the 3 coordinates of a given point A(a1, a2, a3) which belongs to P. So from that, I can easily get the plane equation.

Now, if the 3D projection of A onto the plane gives the coordinates (x, y), what I want is to find the 3D coordinates of a B point (belonging to the plane), assuming that the 3D projection of B onto the plane gives the coordinates (x + x', y + y'). Of course, x' and y' are known.

I'm sure this must not be hard, but I fail to find the solution..