Let’s say Nigeria signed a predatory loan offered by the US to develop infrastructure. The terms of this loan is that they must pay $xxx USD per year until the loan is paid off with xx% interest. The contract will explicitly state they cannot pay in resources or other currency besides USD (and related bonds) in order to create dependence.

China could offer a loan that gives Nigeria Treasury bonds equal to their total loan to the US, allowing them to pay off their loan to the US in exchange for less total debt to china.

This would change the debtor, but also usually the terms so Africa can now pay the debt however they choose, rather than a specific currency.

Or, more simply, Bob loans Jim twenty Bobbucks and only wants Bobbucks back in the future. Chen has twenty Bobbucks and want Jim’s help in the future, so Chen gives Jim the Bobbucks in exchange for a favor or some moneyary debt that isn’t Bobbucks. Solving the issue.