Open MC's phone, click on the WiFi/singal bars, select 'Player'. From there you can add money $1,000 at a time to your heart's content. (I don't know if this is Cheat mode only, of if Cheat mode just pre-loads you with stats and monies and allows you to skip minigames)
Or once you get to the Nadya/mafia route, put all your money in the bank and only withdraw the amount you need to do a specific thing (i.e. buy wardrobe, get gifts, buy vehicle, purchase beach house, etc) as and when you are given the prompt/option to do so; take that amount out of the bank, do that thing immediately, then re-save the game.
Maybe I'm mis-remembering, but I feel like every time it happened to me when I first played that route they would take 100% of whatever money MC had in his wallet at the time of the encounter. I always went the whole way down to zero$. Then again, I don't even remember what I had for breakfast yesterday, so who knows.