Problem, in Transfering of a material

Dear Guru's,

I was trying transfer a material from one storage to another storage location in back date on 31.03.2008,The posting periods are still open.

But i am facing an error

"Deficit of BA Unrestr. prev. 2.630 L : 204200549 PLBW BRMS 31030108

Message no. M7021"

Pls tell me solution