Filing st3 amendment

Dear Sir

We run a franchise of International company and while providing service whatever amount is collected 20% of amount collected( and ST on 20%) is returned to the company. we pay service tax on 80% and company on 20%.

We are filing Service Tax on our own from last two years and we are paying ST on only 80%.While filing ST3 we mentioned collection figure of only 80% for each month. We did not filed on 100% of collection and took cenvat credit on 20% ( Pls note company paid taxes on 20% and provided us Invoice for that).

Do we need to file ST3 ammedment, if so how to do it?

Thanks for help.