At 8% of the 2019 GDP, a lot of that $36B is going to eventually make it back into government hands in the form of taxes.
In 2019 the Nigerian government’s budget was 3.4% of the GDP. If that $36B gets taxed like the rest of the economy it’ll raise $1.22B for the government. Even assuming that fraudsters are dodging their taxes at a higher rate, the tax revenue alone from $36B might outweigh foreign aid for the year.
Also consider the Nigerian government current receives aid money, plus whatever amount they're able to tax from this fraudulent money (which is probably pretty low as you mentioned, fraudsters are unlikely to be on the up-n-up with their taxes, especially when the income came from illicit activities).
So chopping nearly $1 Billion in aid is going to hurt badly, even if the fraud continues as normal.
In 2019 the Nigerian government’s budget was 3.4% of the GDP. If that $36B gets taxed like the rest of the economy it’ll raise $1.22B for the government. Even assuming that fraudsters are dodging their taxes at a higher rate, the tax revenue alone from $36B might outweigh foreign aid for the year.