CRM: Billing document lock exception in CRM_ORDER_MAINTANE Fm

Hi ,

In my project there is one requirement of creating 2 line items in one billing order one is for rejected status and other is for cancellation of invoice.

i am using 2 function modules in which order_read and order_maitane FM is been called.

After calling first FM it is successfully adding new line item for rejected status but after that when calling other FM to add new line The FM order_maintane is throwing exception as document locked.

I have also seen in SM12 and found lock created by first FM called.

Please help me unlocking the document through program.

Thanks