Change SY-UNAME

i need to change the user name before calling a function module which releases an invoice frompayment block.

actually scenario is a user will release the blocked invoice using the bapi BAPI_INCOMINGINVOICE_RELEASE.

But he has no authorization to release the invoice.

So can i change the user name just before calling the function module so that the new user id will be used for authorizatio- check.

i have tried to do this.but the user name being reset to the current user in each step of execution.

is there any way to chagne the user for security check?