GR/IR account postings

Hi SAPers.

I am testing a scenario here whereby,

I make a PO of 1500 MT with a net price of 1183 per unit.

then I do gr for 750,250 and 500 respectively.

then a different user does the MIRO but while doing MIRO for 500 he keys in 5000 instead of 500 in quantity.

My invoice price amount remains the same but my GR/IR  gets debited with 1,116,293.83,price diff cr 524,793.83 and vendor Cr 591500.

I am also able to pay the vendor the full amount for the 1500 MT as per the PO price.

Why was this possible and how can I resolve this?

Regards

JM