If your husband at least read the line he signed on and it was marked "witness" then everything is okay. If it didn't say "witness", then he could have signed anything (since he didn't read it), and you may very well be co-signors to something, or given away your house.
Having witnesses sign something is pretty common, and I have had my neighbors sign as witnesses before for myself.
It does sound strange that the nieghbors story would keep changing, and it seems suspicious. You need to talk to a lawyer, and you should have done that already. Most contracts have a time limit to back out after signing, but that is three days in which to cancel. And of course, you have to know who to cancel the contract with, which your husband doesn't.
This was a pretty bad mistake on your husband's part if he didn't sign merely as a witness. Even as a witness he should have read enough to see what he was witnessing, whether it was the person's identity, or the performance of some act (such as witnesses to a marriage ceremony), or the length of residence in their home. There are lots of scam artists in my area, and many of them are trusted by their family friends and neighbors until they slip away with the money.
So, ask your husband if he is sure that the spot he signed was marked "witness". Then he is only liable for mis-witnessing at the most and not the contract/deed/credit application he may have signed. Not that documents can't be altered after the fact, with the witness word painted out and replaced with some other designation if these are serious scam artists willing to do anything including forgery.
In the future your husband should refrain from signing anything without giving it to you first. If he does, you need to make the arrangements as are done for the feeble minded where their signature on any document can be rescinded on your order. Not that I am saying your husband is feeble-minded, but this is pretty dangerous activity to sign things you don't read. He might have signed off on the deed to your house for all he knows, or ordered an inground pool to be built for your neighbors with the bill coming to you. You will never know unless you get a copy of the document, or an eviction notice, and you might need a lawyer to get that now if your neighbors don't want you to have it.
If your husband did sign as something other than a witness, you are going to need a lawyer to get out of it, since so much time has already elasped. The sooner you get on that the better.