Find another agent. It is not routine to get a cheaper fare at Carnival. In most cases the fares should be the same. Carnival may have specials for a particular cruise, but they are supposed to be available to agents too. Also, agents can often access group space which can be cheaper than an individual booking. Many agents will reserve a block of cabins (group space) and can then offer these are good rates.
I'm thinking if this agent offered to let you make payments over time, he is padding things. The normal method is to make a deposit and a final payment. Group space can have smaller deposits, then sometimes another payment or payments, and then the final balance.
What cruise and date are you looking at? I'd like to do my own price comparison.