Sweetheart deal
From ACT Wiki
1. Tax.
A tax arrangement which is unusually favourable for the taxpayer.
2. Tax.
The settlement of a tax dispute on terms which are unusually favourable for the taxpayer.
3.
Any contract or arrangement on terms which are unusually favourable for one party.
Depending on the context, some sweetheart deals may be illegal or unenforceable.
The use of the term implies unfairness toward others who were not part of the sweetheart deal.