Slope under fence

[Tasnate]

There's a slope between my garden and next door's.
My garden is about 4in higher than his.
The fence between is level with my ground, and soil washes underneath into next door's garden.
He isn't happy.

Without replacing the fence, how can I fix this?
A) Dig underneath the fence and fill gap with bricks/concrete
B) Put in a lowered flower bed level with next door
C) Something else

 

[Bouy]

A) or gravel board

 

[Tigercubrider]

Concrete gravel board.
My garden is similar and my fence is a 12" gravel board sunk to 6" on my side. Then wooden panels all with concrete posts
I remember my Dad doing similar in the 70's but he used paving slabs on end.

You could simply dig a very narrow trench on your side and add some kind of solid barrier like bricks on end, paving, gravel board without dismantling the fence