Hi,
Unless I miss something, but system's behaviour looks correct to me. If FM year is determined by delivery date, then invoice is trying to consume the budget from 2015. However, the commitment of PO is still in 2014, because at the time of their creation, FM year was not yet determined by delivery date. What you have to do is to carry-forward commitments of these PO's to 2015. In this way, the commitment will come with the budget to 2015 and invoice won't consume supplementary budget.
Regards,
Eli