In terms of payment got the installments lets take for e.g.. 4 installment total amount is 1,00,000 if that is the case in configuration I can mention that equally 4 installment 25% each (25,000*4) or 1st installment 10%, 2nd installment 20%, 3rd installment 20% and 4th installment 50%.
These things can be configured in tcode obb8 and obb9.
But client is asking let it be 4 installment but what amount to be posted for each installment we won't be knowing only at the time of business deal, specific to customer decided what amount to be taken as installment.
Here main problem is client knows the no. of installment but does not know what percentage of each installment, without which configuration is not possible.