What is the liability when a debtor states, 'I owe you two dirhams, but rather one dirham'?

Primary text

If a person states, 'I owe you two dirhams, but rather one dirham' (lahu 'alayya dirhamān, bal dirhamun), the greater amount is obligatory. This is because the debtor has retracted (idrab) from one amount after admitting it, and this retraction is not accepted. This differs from making an exception ('illa'), as an exception does not negate what was admitted but rather defines the remainder, such as saying 'ten minus one dirham,' which means nine.