Commitments values

Hello Experts,

I am presenting my issue in simple layman terms. I create a purch.order with net value of Rs.100/- and assign it to a wbs element (project systems). when i use creditable tax code in the p.o with say service tax 10%, the system shows commitments value against the wbs element as Rs. 100/- i.e. the net value, whereas it shows Rs.110/- ,i.e. gross value, as the commitments value when i use non creditable tax code. Are there any specific settings for different tax codes by which system determines the amount ?

Regards,

Manish