FB60 payment method when splitting amount

Hello

We have a problem in transaction FB60. When a document is created using this transaction, we need to implement a check

on the payment method. To achieve this, we use a FI substition (OB28) on field BSEG-ZLSCH. But when we split the amount (strip 'Amount split)', BSEG-ZLSCH is cleared and we cannot made the check. How can we fix that? Which field(s) contain the value of the payment method when amount is split?

Thanks and regards