Hi Pedro,
If you don't want to pay the retention money to vendor, then it should be transferred to expenditure GL(credit) or otherwise consider as income and transfer to revenue GL
The entry would be
Vendor Spl GL --------Dr
Expenditure GL-------- CR (In this case your expenditure will come down)
(or)
Revenue GL--------CR
you can use transaction code F-51 or otherwise you can simply pass a JV in F-02
Hope it will be resolved..
Regards,
Avinashh