Adding return in basement

I recently had a 2.5 ton carrier system installed in my approx. 1350sqft ranch. The finished basement is about 380 sqft out of the total 1350. We have one supply going to the basement and it is not cooling or dehumidifying the basement to a comfortable level. As mentioned in my previous post the contractor is coming out on 7/9 to fix some other initial istall problems and I would like to be able to ask them about adding a return from the basement.

First, I spoke with the salesman and he said adding a return in the basement would not help and possibly hurt the system because it would be sucking up the cold air from the supply. Does this make sense? My thought is that the basement needs a few more supplies and a return. It seems like the main return is the hallway is not getting the air up from the basement. I leave the door open.

So, would it make sense to add a return to in the basement or would just adding more supplies be better or maybe both.
Again, does it make any sense about what he is saying about
the basement return being bad. Another contractor was going to add a return in the basement. I'm confused.
The salesman did do a manual J load.

Thanks in advance. Also, thanks for everyones previous responses

most definetly need a return installed and maybe an additioinal supply. hard to say w/o seeing. a low installed return will help "pull" the heat which comes from the ceiling, helping spread the air more equally throughout the space. also returning some of that stale air into the system so it can be filtered. be nice to have a little filtration down there. returns also help balance the room pressure. no returns means your pressurizing your room. not usually a problem but can be in some situations. leaving your door open allows all the cool air from your house to fall to your basement. heat rises cooler air which is less dense sinks.

I agree with coco. Add a return to the basement. when air is moved through the return and across the coils, humidity is removed. Can't say for sure on the supply but if most of the basement is underground, one is probably fine. Two with manual dampers wouldn't hurt.

Important to avoid depressurizing the basement. Check undercut on door at the stairwell. Negative pressure backdrafts chimenies and sucks soil gases/moisture into basement. Is the basement hot or damp?

Thanks for the responses. The basement is both hot and damp.

Remember air conditioning removes heat. With no return this function is not possible.