How To Sell My Downpayment Of Condo And Buy Another Condo?

Hi I have 2 questions about buying and selling the installments paid on an uncompleted condo building:

1) i bought a unit direct from the developer, say unit No123 at ABC Condo for 3 million baht. This condo will be completed in 2 years' time. The downpayment is 450,000baht and can be paid by installments. So far I've paid, say 60,000baht. I have already transferred 1.5 million baht from overseas, and got the letter from the bank that says this money is for buying unit No123 at ABC Condo. Now if I want to sell it, how do I go about?

2) I would like to buy another unit at the same ABC Condo building, but another unit number, say No567. But this time the seller is a person, not the developer. He is selling this unit at 3.1 million baht and has already paid, say 80,000 baht worth of downpayment. How do I go about buying this unit from him since my overseas bank transfer stated that the money i brought in was for unit No123, not unit 567.