Retention or holdback in sap mm

Hi

I have an urgent issue of setting up retention senarion to my client.

can someone  give me step by step procedure of retention in  ECC6.0 ehp 5

i have 90%  to pay to the vendor while 10  holding back  need to be paid after 6 months.

but when i post the MIRO systm should keep this 10% in special G/L .

It is very urgent and thanks in advance for helping me.

regards

Shahab