Service PO value amended after invoice is done

Dear All

I have scenarios and I want to understand how this has happened. My understanding was system would not allow this.

The PO was created with an AED 10,130,000.00 and invoice posting was done for AED 10,125,956.70 and the balance to be paid AED 4,043.30.

But when I tried to amend (reducing) the service PO value to 10125956.50, the systems allow me to reduce 35666 AED.

PO  Value   : 10,130,000.00

Invoice Created  : 10,125,956.70

PO value amended to 10,094,332.14

Could you please advise, as the system should not allow values to be deducted for what has been paid.

Regards