I am creating a revenue schedule for an item. The total payment is received
over a period of 24 months in 10 installments, each of variable amounts, paid
on variable times, starting from the date of sale (variable again!!!). All
the items are being sold over a 10 years period. I want to be able to
calculate monthly payments (for 10 years or 120 months).

This is how I was doing it for 5 installments:

=IF(Month 1<Sale Date,"", IF(DATEDIF(Sale Date,Month 1,"m")=FI time, FI
amount, IF(DATEDIF(Sale Date,Month 1,"m")= SI time, SI amount,
IF(DATEDIF(Sale Date,Month 1,"m")=TI time, TI amount,IF(DATEDIF(Sale
Date,Month 1,"m")=FoI time, FoI amount, IF(DATEDIF(Sale Date,Month 1,"m")=FiI
time, FiI amount))))))

where,
FI: First Installment
SI: Second Installment
TI: Third Installment
FoI: Fourth Installment
FiI: Fifth Installment

The above formula is used for 120 months i.e. from Month 1 to Month 120.

Now my problem is that I can't use the same formula for 10 installments.
Can somebody help me please?