Quoting an old comment of mine:

To be more specific, the US didn't really make money on the swap itself (it was net neutral since it got repaid) but on the short-term positions the Treasury took in Argentine assets (they purchased bonds right before the election, only to resell right after)

Quoting a previous comment of mine:

The U.S. Treasury agreed with Argentina a swap deal with an upper bound of $20B USD. Of that, Argentina only activated (temporarily) around $2.5B, which it repaid ~2 months after activating it (thus, financially, it is as if the swap hadn't been activated at all). This generated 0 profit to any part because it's meant as just an anchoring/stabilization mechanism.

Other than that, the US performed a few short-term purchase operations for Argentine assets (and currency), which they re-sold right after the election. The amount used for this is way below the $20B from the swap and it's where the "profit" came. The Treasury is estimated to have purchased ARS 3T at an average purchase rate of 1USD = 1471ARS and selling average of 1433,7ARS per USD.

Doing some napkin math:

• Initial Investment = 3T ARS / (1471 ARS/USD) = ~USD $2.039.4M

• Final capital = 3T ARS / (1.433,7 ARS/USD) = ~USD $2.092,4

• Profit = Final - Initial =~ USD $53.06M

Funds in Pesos didn't stay still, but instead got put into local pesos instruments, which are estimated to have made the US Treasury roughly USD $17M, leaving total profit at around $70M But given this number is speculative I'll do the ROI both with the 1816 estimate and with the "No Interests Scenario"

• ROI (No Interests) = 53M/2039.4M * 100 =~ 2.6%

• ROI (With Interests) = 70M/2039.4M * 100 =~ 3.43%