Return value only in rows specified by entering a number in an exterior cell to the table

BRaff 

Twifoo's solution is elegant and compact but relies upon hard-coding both the extra payment periods and amount within the formula.  To avoid this some way of parsing or searching the comma-separated string is needed.  I chose the latter, first padding the list with additional commas to give a name ONE.TIME that refers to

= comma & ONE.TIME.LIST & comma

where 'comma' refers to 

= ","

(just to improve legibility).

The test to determine whether the payment number has an associated one-time payment is

= ISNUMBER( SEARCH( comma & amortization[@[PMT NO.]] & comma, ONE.TIME) )

which is held as a named formula 'ONE.TIME.NO?'.

Finally to populate the 'EXTRA PAYMENT' column (your capitals, I am not shouting at you :-)

= IF( ONE.TIME.NO?, ONE.TIME.AMOUNT, "" )

Another catch is that the user may embed spaces in the list of one-time payments, in which case an extra processing step is needed to replace them with null strings.