I have been paying 100.00 per month to a buy here, pay here car dealer since last Dec. toward an outstanding balance of 1700.00. Suddenly in May the dealer tried to refuse my payment because he wanted more money. I verbally reminded him of our agreement and he accepted payment. In June he again refused my payment. I sent a letter reminding him of our verbal agreement along with a check for 100.00. He has not cashed the check. I am getting ready to send my July payment along with a follow up letter (the first letter was sent certified mail and this letter will be as well). If neither of these checks are cashed or if this payment is returned to me can I file a notice of default and how do I prevent him from repossessing the car?
Thank you!

"A verbal agreement isn't worth the paper it's printed on." ~ Quoted from memory because I don't recall the original speaker.
You can Google "verbal agreement laws" in your area.