When Can the US President Forgive a Sovereign Debt?

Let us assume that the US has made a loan to a foreign sovereign for some combination of political and benevolent reasons. For example, to support some friendly nation after they get hit by a severe hurricane or to help out a military ally with arms after they have been suffered an unprovoked attack by another nation.

Congress has the "power of the purse", so loans have to be approved by it. But does Congress also retain the power, at some later date, to decide on whether some portion of this debt can be forgiven? Or can the Executive Branch make the decision here? At first cut, under the "power of the purse" rubric, my thought was that surely Congressional approval would have to be obtained. But a fascinating new paper by David Del Terzo, appropriately titled "When Can the President Issue Foreign Debt Relief"  suggests that the answer is more complicated.

David's paper is going to be more than slightly relevant to debts owed to the US by, for example, Ukraine, for some of the support that has been provided for the war against Russia that was not in the form of outright grants.  But going back in time, it is perhaps also relevant to loans made to the UK from WWI. Doesn't look like the US ever plans to collect on those old debts. But they are still on the books. Could the President buy some credit from the UK by generously wiping them off the books?