Customer payment in F-28

Hi experts,

This is the typical process we are following now.

Lets say i have three invoices 1000, 2000 and 3000. Received check from customer for 5700 and 300(100 on first invoice ,100 on second invoice and 100 on third invoice) is deduction due to many reasons.

I will go to F_28 select these three invoices and post deduction with reason code, so this not picking invoice number(blank) in deduction line item.

After posting this if i try to put invoice refrence number as a part of applying deduction amount to individual invoices its giving document "xxxxx" contains no suitable line items(message number F5347).

Help me how can i solve this.

Reagrds,

Lucky