Hi Markyd, i'm no expert but was in a similar situation with finance company, i had paid more that half of the total payments and there was a clause in the agreement that if this was the case then i could cancel the agreement and give the car back with no further costs or infringments on my credit raiting, i did this and have had no problems since, the car would of defenatly not sold for any where near the outstanding balance.
I would check your original contract and see if there is a section about this.
Also if i'm not mistaken my brother did this as well and had no problems.