If you are not sure you have not paid the amount in full, and it is a lingering doubt in your mind, then you need to clear your conscience. The Prophet (peace be upon him) said, "Leave that which is doubtful in favor of that which is not doubtful." (Reported by Tirmidhi on the authority of Hasan b. Ali b. Abi Talib). According to the rules of jurisprudence, certainty cannot be lifted by a doubt, and that, in case of doubt, we are to act on what is certain.
So, if the doubt is that whether you have paid in full, then you should assume that you did not, and give accordingly until you are certain. If you happened to give more in such a case, it is not considered interest, for we are allowed to repay those we owe better- if we wish to. The Prophet (peace be upon him) "once purchased a camel on condition that he would give him another later. When it was time to repay the man, he told Abu Rafi to pay him back from one of the herds that had arrived; Abu Rafi came back saying, but I find them all better than the one he gave you! Then the Prophet told him, 'give him from one of those better ones, for the best Muslims are those who excel in repaying their debts." (Reported by Bukhari on the authority of Abu Hurayrah).
So, it is best that you pay him more rather than less than what you owe him.