You could just manually write a check for the $80.00 and never enter it on DBA  and then remember there is two checks when you reconcile your Ch acct.
 
But heres my first thought:
( assuming the vendor already has the check for the $240.00. )
1. Go in and void the check.
2, Pick and pay only $240.00 and print a check on plain paper using the voided check#  (this would make your DBA records match the check the vendor already has)
3. Now on a real check pay the remaining $80.00
 
You asked how?  
It sounds like to me that someone printed a check for $240.00, but failed to tell DBA  "YES" that it had printed correctly, and then  "listed it to the screen" for $320.00 and accidentally confirmed that the listed check printed okay.