Okay because you’re new I’m going to assume I’m not being trolled.
The gold necklace is £200. Eric has paid £100 for a silver necklace, which he gave back to his sister in law. Rather than refunding him, his sister in law retains the £100. if he gives her another £100, that means he’s given her £200 and therefore should be able to receive the gold necklace in exchange.