Wouldnt do it.
Rays have a high bio load. A 90 gallon is not big enought to handle that amount of bio.
Now if they are telling you that a saltwater ray is acclimated to freshwater, there is a problem. It wont live. Besides the fact it will put off even more bio being in a freshwater tank. Sounds like to me they are just trying to make a sale. Not worried about the ray at all.
If you really want a ray, please get a bigger tank. Rays are cool to have, but you have to be ready to care for one. Its not a $5.00 trow away fish (no fish is really), its a care takers job. Not something to take shortcuts on just to be cool and have one..