Referencing Jack and The Beanstalk, ‘holding my beans’, means I’m saving my money and not spending it, in the hope of growing those beans into a lucrative beanstalk.
It can also be used to say that money is invested somewhere.
“Are you coming out after work?”
“No, I’m holding my beans.”
“Have you got a lot of savings?”
“No. My investments are holding my beans.”