Hi Mayumi,
The big question is which posting date you have used at the invoice posting (2014 or 2015)?
The possible reason why you don't receive a budget error is that the availability check is only executed if the assigned value is beeing increased. If you have forwarded the commitment to the fiscal year 2015 and execute the invoice posting in 2015, the assigned value is not beeing changed -> the availability check is not going to be executed in this case (see note 783161). There will be no budget check at the commitment carryforward. If you now reverse the Invoice posting, the commitment is again beeing rebuild. If no budget is available then, the budget error message will be justified.
best regards
Marinko