Invoice block release

Once the goods receipt occurred that the standard SAP invoice release program would release the MM document payment block. There are quite a few times we do not get GRs (Samples and Promo) and these MM invoices do not get released.

For the above requirement currently they are manually relasing invoice block using MRBR transaction, but I need to automate the MRBR process by new custom program, I can get all the MM blocked invoices but how do we relase the blocks?. What transaction can be used to release this blocks?.

Valuable inputs will be awarded, appreciate your help