I am not sure if I understand your problem.
Two functions exists in IFS which can be usable for you
- Write-off code, when used in customer payment, closes AR open item to PnL Account indicated by PP17 posting type.
- Using Bad Debt Management (separate window under Manual Customer Payment menu), you can transfer AR open item to doubtful client account and create bad debt provision in parallel (using PnL account indicated by PP73 posting type)..
Normally if bad debts exist in the books during migration to IFS accounting system from old one, we are replicating bad debt process on migration date (creating provision again and adjusting PnL entries by manual voucher). Please note that bad debt is still and AR open item, to remove it from the books write-off function should be used.