Hi Dilip,
To make the Main/Sub of installment plan to take part in invoicing account maintenance, the solution mentioned by Ivor would suffice.
However, if you want that main/sub not to get cleared with a specific credit sub, you will have to enhance your clearing variant.
There in the selection of Main and Sub you can first allow all other transactions to offset (consumption charge and the credit you mentioned), you can achieve that by rank order of Main and Sub. In a subsequent step you can put a logic (FM *TFK115 - (customer FM) or configuration) not to offset 0080/0010 with the credit main/sub.
Hope it helps.
Regards
Bodhisattwa