The buyer low-balled the seller by offering only $500,000 to test their bottom line.

low-balled 是动词 low-ball 的过去式形式。low-ball 由形容词 low（低的）和名词 ball（球）组成，原指棒球比赛中投手故意投出低球，后来引申为商业谈判中"故意报低价"的策略，目的是试探对方的底线或迫使对方降价。