BCS from PO

Hi All

I have 2 question. I use period wise budget using fund ( I created 12 fund for each month ) and release budget from FMMPRELE. Our fiscal year is from January to December. If I created a PO in 15.01.2015 with delivery date that has 20.03.2015, which month's budget reduce???? January?? March ?? I think budget of March reduce in case. If I am correct, When GR is coming inventory account will be updated and budget will reduce again. We derive DUMMY commitment item and fund center for GR/IR account.

question 1 - Does BSC check same purchasing document number up to the IR? I mean the system reduces only one time for same purchasing document.

question 2 - Can you explain how is working BCS above case?

Thans a lot

Yasin