Dealer causing me stress asking for money

I recently bought a used vehicle from peak Kia. I was told that I would be given 3200 on my trade in . I agreed to the terms and signed the paper work . However the very next day the dealer called me back to notify me that the bank that agreed to finance me back out if the deal. I ask if the deal would change through a new lender and they replied "yes" I asked if there was anyway that the deal could be canceled because I cannot afford to pay anymore money monthly that we previously agreed on. I was told that I do not have a choice because they had a signed title for my car/trade in and that I was not going to be given my car back so I had no options but to sign whatever deal they gave me. We'll we came to an agreement, this was two weeks ago but now I have them calling me and saying the bank wants more money down. I don't know if any of this is legal for them to harass me stress me out or strong army like this and I am also afraid that they're going to charge me $50 per mile per day