you're not overreacting a single bit.
not at all.
that is totally ridiculous that she offered to buy something that you most likely would have to help pay for without asking you first, it would be afterall half yours since you'd have to sleep on it and stuff.
and like you said, you have a new set of furniture already, there is no point in getting rid of it especially when money may be tight.
Yes I understand there has to be compromise in relationships, I practice it quite well in fact, but I believe you definitely have to put your foot down on this one.
this is a materialism vs. practicality issue.
wants vs. NEEDS.
stand your ground and refuse to budge.
(if I find out you let her change your mind with her female tricks, I'm gonna drive over there and kick you in the ass!)