Let me be more precise. If I promise you 6 and pay you 5.5 but owe you only 5 have I wronged you?

No one is wronged unless they are not paid what they are owed.  By not keeping a promise you appear to be less than trustworthy but you have not cheated anyone.

yes, cause you didnt pay all of it like you said you would

No, now keep that promise and gimme those doughnuts...5.5 is not bad;))

I'd say if you only owe 5, perhaps they owe you some, or perhaps you have already overpaid on something else... then you haven't broken your word, providing you give a clear and understandable explanation of why you only paid 5.5