ml81n service entry sheet amount modifiable?

Dear All,

Request pl let me know is there any possible of modifying the amount in ML81n, while doing accept & Save.

I have a Service PO value Rs. 1000/- for one service activity, after the work completion we have to modify amount in ML81n based on the Bill, because Vendor bill is for Rs. 900/-

When I do ML81n, amount is going grey and cleared for Rs.1000/- but Rs.100/- shall be lying in GR/IR account because of PO value Rs. 1000.

Anyway MIRO is done for Rs. 900 based on the bill, but how to knock off this Rs. 100/-

Pl suggest some solution.

Regards

Srihari