No way in hell any banks will let that happen. Banks will usually determine your loan amount based on the price reflected in the sale and purchase agreement and not the valuation report unless you borrowed the extra RM50k in another form and not for the finance of the purchase of the property (eg: renovation etc). Some banks still loans for renovation.

Owner can mark up the price but nowadays, not many solicitors will dare to do so as it amounts to cheating and recently, enforcement is rather tight... for who knows why. If done, usually the owner will execute a letter of rebate/discount addressed to the purchaser and deposit it with the solicitors to be utilized for the release upon completion.