Service tax credit

Hi,

Abc a partnership firm is a works contractor. It issues bills Rs 100000 to Xyz a builder with service tax @ 4.94% Rs 4940/- total rs 104940/-

Abc is charging service tax from customers for flats @ 3.09% on Rs 100000, It receives Rs 3090- Total Rs 103090/-

Now while filing service tax return,Xyz wants to adjust Rs 3090 (Service tax payable)with Rs 4940/-(Service tax credit) and want to carry forward Rs 1850/-

But the system does not accept the same and does not show c/f of service tax to next period

Pl help!