budget check in production order

Hi Gurus,

  i have created project and maintained budget also...

   for example:    i have given budget for wbs element below. 

i have created Purchase order for rs.60 then i received rs.30 available budget is rs.40.

Then i am creating Production order it's asking budget. it's through error message for budget.

i have doubt in production order why it's checking in available budget. if we create production order when budget has to be consume whether it is production order creation time or confirmation time.

please suggest.....