Original Question: There are times when the seller says “I am selling it to you for S$20” but the buyer says “I am buying the item for S$15” is this transaction valid?

Nope, this transaction is not valid unless it is still in the negotiation phase which is still valid.

But in negotiating, the buyer must take note that he shouldn’t oppress the seller by pushing the prices so low as the seller too has the right to profit and pay off his business expenses.

Yet, in this case, the conclusion of the deal (sighoh) must mention the price agreed by both parties as not doing so only shows that the price is still up in the air, causing the transaction to be void.