The prospective buyer couldn't really understand the terms of the contract.
Literal
Promising-[attributive-な] buyer [topic-は], contract-contents [object-を] well understand-potential-not-[polite-past-ませんでした].
有望 ('promising, hopeful') combined with 買い手 ('buyer,' literally 'buying hand') gives 'a promising buyer' — a prospect likely to close the deal. 契約内容 ('contract contents/terms') is a formal compound common in business Japanese. 理解できる is the potential of 理解する ('understand'). The whole sentence is business register.