Dsum and iif on report

Experts,

I am trying to combine the below in an unbound field on a report.
Maybe its better to do it by VBA?  
Not sure which way is easier.

thank you...

In English:
If tblLetterOfCredit.PaidInFullYN = True then "Paid In Full", if not then Dsum but only if tblPayments.PaymentDate is not null (meaning there is a date entered)

--------------------------------------
If tblLetterOfCredit.PaidInFullYN = True then "Paid in Full"
Else:

If is not null [PaymentDate] then Dsum [Amount].  How to add Is Not Nulll to below?
=iif(DLookUp("[PaymentDate]","tblPayments","[LCID] = " & [LetterOfCreditID]
dsum("[Amount]","tblPayments","[LCID] = " & [LetterOfCreditID]