Its been resolved..
For budget not being available for consumption after being posted through FMBB th issue was in my AVC profile. and for the second part where budget was not consumed after invoice being posted through FB60 the issue was in my fiscal yeas variant year shift. the posting was taking plane in the wrong fiscal yeas hence consuming budget for that particular fiscal year and not of the year i intended to post to. Thank you so much fpr approving my query 
Regards,