Football debts are pretty safe because of the sanctions fifa can I lose for non-payments so there must be more to it.No links but remember there being complaints about it in the year or so after he went across. Can't remember the context entirely as it's a decade ago now, but remember there being some beef on being slow on payments.
Maybe he was just joking about an unfavourable exchange rate or inflation fluctuation which could make sense. But Both of those can still be addressed in the contract without needing to demand a massive majority of the fee upfront.