This sounds like a data issue. Minnesota FC lists Hlongwane as a U22 Initiative player, not a Designated Player. A designated player under 24 carries a salary cap charge of $200,000 p/a, or $150,000 if they are under the age of 20. But there are ALSO U22 initiative slots and as long as Hlongwane's salary is below the maximum budget charge (which it should be, because in real life he's not a DP) that makes him a U22 player which has the same salary charge as a Young Designated Player. Minnesota should have 2 additional U22 slots available.
https://www.mnufc.com/roster/
You also could (at least IRL) use allocation money to buy down either of Amarilla or Reynoso's contracts so that they weren't DPs anymore.